RpNation

It is a courtesy. I usually like any status message that interests me enough to go to the person's profile page and look around. I also like all of my partner's RP posts, since they put the effort into replying to me. For general threads, I also like something if I was interested in what someone said.

Oh, just thought of another reason! I know some people who do it to let someone know that they have read it. I do that sometimes.

So it's an interest-courtesy thing. Hmm...