This is along the same lines as what Leebo said, but what I read on the Wikipedia page about keigo the other day is that when you use humble speech, the actions that you’re describing need to have a target which you elevate. Also, there are technically two types of humble speech (謙譲語):
- (Typical) 謙譲語 – the person doing the action (you) is lowered relative to the target/subject-matter. For example, 伺う or 拝見する. You make your actions respectful towards the target.
- 丁重語 – you describe your actions and things related to you in a respectful way to the listener. The target of your actions is not necessarily of higher status than you. For example, 参る. You show respect to the listener and may also lower your own status through expressions like 拙著 (‘[my] humble book/work/piece of writing’; 拙 is literally ‘bad/poor/unskilful’) or 愚息 (‘[my] foolish son’).
As a result, even when talking to a friend, you can say 「上司のお宅に伺った」, and when talking to someone respectfully, you can say 「弟の家に参りました」, but you can’t switch 伺う and 参る in those two sentences because
- for the first, you need to show respect to your superior at work, and it would be weird if you show keigo-level respect to a friend. (It’s too formal/distant. He/she might feel it’s 他人行儀 – he/she’s being treated as a stranger.)
- for the second, you can’t elevate your younger brother relative to you because he’s part of your うち
尊敬語 is similar to the first sort of 謙譲語 in that it elevates the person doing the action relative to you by raising their status.
Here’s a summary table of the five types of keigo, with examples and the people to whom they show respect, along with an organisation chart:
Source if you want to see where I got everything I just said (and the images) from: 謙譲語Ⅰ・謙譲語Ⅱ（丁重語）ー日本語教育Wikipedia
Random fun fact that isn’t really relevant: there’s also something called 尊大語. It’s not 敬語. It’s language that’s used by the speaker to raise his/her own status. 参る can be used as 尊大語 if used to command someone else to go/come. I don’t think anyone does that anymore though. A more common example is 俺様, which you hear a lot in anime when a character is portrayed as self-important.