Janet Jackson said it best in the song "Again". I hate that she was able to describe the feeling so well.
How come even after years of not being with "that" person, when you do see him/her you can feel all those things rushing back again? How is it that no matter how much you hate them, or how much they hurt you, you keep falling.
It's upsetting, it's irrational, it's hurtful, and yet, we keep on allowing the feeling to take over us and control most of our body and mind.
It just sucks!