Why am I commonly accepted as an acquaintance but rarely as a friend?

This doesn't really bother me, since I'm something of a loner anyway, but I just wonder why. I'm generally well liked, I'm always polite to people, treat them nicely, can hold a good conversation. I've been told that I'm pretty funny as well, and people seem to open up to me naturally once they get to know them, which shows that they trust me. But I'm rarely sought after beyond school or work or wherever I meet someone, I'm generally kept in that box. Just wondering what I might do or not do that garners this reaction?