Think of someone you love; someone who you're certain you could spend your life with. Now imagine that they smoke, or have some other habit that you just absolutely can't stand. Do you still love them enough that you'd want to spend your life with them?
That's an interesting threshold, at least for me. There are few people I know who I think I could spend my life with as it is, but with the gotcha that they smoke, that number drops to maybe one or two. It's something that I've really not thought about until now. It's nothing inherent to smoking; there would be several habits that would make someone similarly unpleasant to live with. There are people who love those who are chronically untrustworthy, or who cheat on their loved ones, or are alcoholics... and there are those who thought that they loved someone before those habits developed, and found that they no longer do.
Anyway, I thought it was interesting how one little everyday habit could so dramatically affect something so important and profound.