Personally, I will sub a channel for a few reasons. One is mostly because the content interests me. Also, I will sub as a show of support to someone who is supporting me. They post a new video, I comment on it. I spend about 2 hours a day just watching things that I've subbed to show my support. But I'll notice that over time, I am spending a lot of time commenting and giving support, but that person has yet to do the same. I don't question them about it as I don't want to sound like I'm begging for comments and likes, but I do feel as though if I support you, it would be nice to get the same. But once I realize the person has no interest in what I'm doing, or simply isn't showing support in return, I simply stop commenting.
But after a while, I'll notice an increase in videos from this person and NOTHING in my inbox saying they even attempted to check out my stuff, so I simply click UNSUBSCRIBE and move on. In most cases, the person doesn't even notice.