For me it’s mostly an interest in the culture (mostly pop culture). But also learning for the sake of learning, instead of just wasting all my time at home on games/netflix.
Besides those two I’m also planning on doing one or two semester in Japan next year so I might as well start studying now!
I started learning via Duolingo for an internship and then found out kanji were a major deal once I arrived in Japan. (Duolingo was pretty bad back then, no kanji at all). Then I started with Wanikani and reached level 23 once I got back from my internship. I loved learning the language so much that I just kept going and I am planning on mastering the language in the future.
It started when I kept seeing Japanese comments in the source code at work (external contractor for a Japanese company), so I learned katakana to decipher at least what they were referring to. Then I got a little into it and looked up kanji for technical terms. Now I noticed that I will not make significant progress before this job is over but I took a liking to the culture. Hopefully I will someday be able to play videogames in their original language. Vanity is also a factor, although I keep this undertaking secret from my co-workers.
I guess my main reason comes in two halves, but mostly it comes down to wanting to understand pop culture.
Half 1 is that I accidentally picked up enough of the language from watching subbed anime that I started picking up on when there was some context that was really, really hard to translate. Not just formality and style of speaking, but when specific dialects are used or a point relies on an understanding of how a particular term is used. I think this came to a head when I got though most of Akagi without realising a major character’s dialect is supposed to indicate a pretty large part of his backstory…
Half 2 is that I’ve been in transformative fandom since I was a kid, and Japanese fandom is super different to English language fandom in a way I’m not used to. I wanna go read Japanese fanfic and understand the culture better… I wanna buy doujins…
If I want to be all respectable I say I recently realised how useful knowing French has been in my current career so I figured learning Japanese would be even better (I work in tech) but… That’s a lie of omission 
If this were back in college, when I would have fallen more into the weeb category, it would have been for some silly reason like imaging that I’d be some sort of digisubber translator or some nonsense.
These days, it’s really just more of a challenging hobby to have. That there is some research to show that learning a second language helps sharpen your mind against aging and possibly protects against Alzheimer’s is just a side benefit. 
But ultimately, one day I’d love to be able to watch some of my favorite movies like those of Juzo Itami without the pretty horrendous English subtitles that either the US or Japanese release have. And to possibly learn more about the cultural aspects that would add additional understanding to such movies.
It started as an intellectual challenge, but now I just want to be able to communicate well in Japanese
This would apply to other languages as well wouldn’t it? Or am I misinterpreting your comment
I selected pop culture. I feel like I’m your pretty standard middle aged US based anime/video game playing person who has also picked up a smattering of words here or there. I watch most things in subtitles even if its in my own language and sometimes enjoy noticing discrepancies between what’s being said and what’s being written, so partially intellectual as well. Only two languages have ever peaked my interest, with one being Japanese and two being American Sign Language. Given my hobbies and friend circles, I’m more likely to exercise one over the other although it’s a shame time travel doesn’t exist so I could redo my life and be fluent in all 3. =) As more of a part time hobby learner I’ll probably never reach full mastery but hopefully I can enjoy the journey.
No “main reason” but many reasons…
*manga, anime (how it started)
*I had to learn german and spanish…but never used them after school…I tried chinese too…but my brain & ears are not compatible with tonal languages…Japanese is just fine. It also got ride of most of the brain cells used for spanish and german…
*martial arts (to enjoy classes more, books, and to help translate to other languages too..to share and teach better…)
You can learn some moves by watching, but how much better can you get, if you also understand what the teacher is explaining patiently in japanese… (just watching can lead to so many mistakes, if you don’t get the idea behind the move…)
*massage (lots of books and classes in Japan, but too advanced for my japanese ! 
)
*my kids are half japanese…
*to help with finding a job again in Japan, in a few years…
*to help with exploring Japan, when I’m back …
*because…I started learning it 20 years ago, forgot, learnt again, forgot, … and I want to see the end of it !
*japanese women… (it was probably motivating me at some point)…
Now I just owe it to the many people I met in Japan, who were patient with me…I want to be able to chat with them with more fluency…
To be able to speak with my girlfriend more and more.
To get around smoothly in Jaoan.
To implement more Japanese in to my music album releases and videogame I’m developing.
Technically yes, it would. It would be nice to know what some of the spanish-speaking and korean-speaking folk in my community are trying to say by myself.
But I prioritize Japanese because there’s a huge creep of media and culture (especially games and anime) from japan being imported into America and across the world. Many people rely on teams and organizations to subtitle, dub, and localize these things. I fear that if I don’t understand it now, I could be taken advantage of by those who can translate. I have a strong need for independency.
My answer is like a combination of 3.
I do like anime a lot but in the same way I like Disney? I just love animation, and Japan, obviously, do it extremely well and very varied.
I want to travel there, I’m going next year for my honeymoon and would love to appear as if I’ve made an effort to learn the language of a country I claim to be so fascinated by.
I also have always wanted to learn and language, and gave German a go, but I’ve been studying Japanese for nearly 10 months with no slowing down, and with someone of my short attention span, that’s quite a feat!
I am interested in learning Kendo. Much of the instruction is in Japanese. So now I am interested in learning!
I study Japanese because I want to read and understand books.
Both “I like Japanese culture” and the intellectual challenge/learning for its own sake.
Another reason is that I have tried to learn the language before but suffer from false starts where I make a lot of progress but then quit for a while. Working on a long-term project is a test of being disciplined enough to keep trying even after the initial enthusiasm wanes
“to be able to read in Japanese” - no other particular reason.
Twice i’ve been to Japan and on both occasions being only able to read kana and less than 50 kanji I felt like an illiterate and that aspect in my travels was not pleasant, having to pull up google translate every 5 seconds, so the next time I’m in Japan I want to be able to read in Japanese to my heart’s content for the sake of convenience.
Also, it takes forever to wait for scanlations.
Japanese rings beautiful to my ears and resonates with my soul.
Whilst the first option isn’t wrong, there are many reasons.
Media consumption - - there are stories and experiences told in Japanese you can’t get anywhere else.
Communication and culture - - Japanese natives are fun to speak to and you can learn to incorporate aspects of Japanese culture into your life if you feel are beneficial. Taking your shoes off, fashion, etc.
Food - - ordering food from a Japanese restaurant in Japanese is always an enjoyable experience. This lends itself again to #2.
Self gain - - learning another language is good for your brain, especially Japanese being structured more logically than English. It also sound and feels great to speak when you get into the groove and words fly out fluently in a conversation.
Well I’ve kind of gone through different reasons throughout my life.
First, I studied Japanese in high school because I was a teenage weeaboo and wanted to live in anime land.
Fast forward about six years without any study; I was going through a severe bout of depression, and figured the only way I could make life fun or interesting anymore was by living in another culture, so I entered university to go for the JET programme, half-heartedly picking up my studies from my high-school years.
Fast forward a few more years and I came to my senses and dropped out of uni, realizing that I never wanted to study the course I chose in the first place and that I was doing it for the wrong reasons.
Now I study Japanese in hopes of breaking JLPT N2 or N1 and finding a job to earn myself a working visa and hopefully, eventually get residency.
Main reason hard to pinpoint, it is more of combinations of reasons, circumstances that turned out to be what it is now my usual routine among others.
Which happens to lead me in path of learning japanese. Hopefully. I don’t think, i will be able to accurately list and describe objectively reasons of it, because i, probably, don’t remember everything. But here you go.
I used to be very ambitios about learning languages for some reason, i guess, hormones.
I wanted to learn bunch of them like korean, chinese and, among others, icelandic language, but this is another story. English not my native also by the way.
Japanese was the most desireble one, because, basically, in hindside and put it simply , i believed japanese are super heroes and also was heavely influenced by many japanese artwork, like games, cartoons, movies in my very early life. I have some passion about japenese culture. Well, i guess, i would say weaboo in very literal and extreme sense of the word. Grotesque one if you will.
As you can see, it was very, very naive reasons, to say the least. I was very young, you see.
I started to learn japanese seriously way after the fact, that i wanted to learn this language though. Just out the of blue and because i thought it is good for health thing to do.
at the beginning it was very slow, miserable process for awhile and then beta of wanikani happened, it was much easier. i read some material and read japenese stuff since then. And here are am, still continue doing so.
It isn’t my main learning activities, that i trying to put all time into, but I stick to it neverthless.
I learned too much already to stop and I kind of used to learning it in a way, doing it for so long.
Some days i put more efforts, than others though.