So, I just rolled up my sleeves to type this.
I just saw (500) Days of Summer. I feel a bit messed up from it. I once was in a very similar situation.
Quick synopsis: Guy meets girl. Girl is nuts. Guy falls in love with girl. Girls says "I don't want a bf, let's be friends." Guy stays by her side. Girl acts like a gf anyway. Girl leaves, girl comes back, girl gets married. Guy is crushed. Guy meets new girl.
The End. (I promise I didn't ruin it for those of you who haven't seen it.)
But yeah. I've been in this same situation. Several times in fact. Minus the marriage part.
It's all about uncertainty. I can't say I've ever been in a relationship where I was certain the guy was THE guy for me. And I suppose you really can feel something like that. But that's definitely why I always pull the "I don't want anything serious" card. That way, when it all falls apart I can say "well I told you..."
Stupid girls. Stupid, stupid girls.
She's like the wind...