When it comes to commitment, I've never had a problem with it. I like someone and then I only want to be with that someone until the break up comes. However, in the current relationship I am in, I feel that marriage is something I want. I don't think I could have kids outside of marriage, and I want kids, and I want those kids to grow up in the same home as both parents. I don't think that's possible without some sort of agreement between both parties before having a kid.
I agree that communication is the big issue here, and I know that Andrea feels the same way as I do on this subject, so that is part of the reason it's easy for us to commit to one another. If I do meet someone that is better than Andrea (which is possible) I doubt that I'd go through with anything. It would probably be a novelty thing, and while novelty is awesome, it wears off quickly.