Tolerance and acceptance...psychologically, at least. I want to promote the idea that you don't have to think the same way as another person in order to appreciate them.
Not only that, but a friendship between people who are 'opposites' doesn't have to hinge on them picking at each other and constantly trying to make the other more similar to themselves. The social butterfly doesn't have to drag the bookworm to parties against her will, and the bookworm doesn't have to condescend to the butterfly and treat her as an intellectual inferior. That stuff is fine for comic relief...in the same way that slapstick is. I don't think it's supposed to reflect reality.
So the one common thread in most of my stories is forging accepting friendships between people who are so wildly different in so many ways, and just letting them be. Just letting them like people who aren't them, and being okay with it.
Stories about people who meet someone who changes their way of life are all well and good, but I prefer stories about people who meet someone who helps them to realize that they're just fine the way they are. I'd like to see more of those.