There's a need.

"It's a strange thing, how you can love somebody, how you can be all eaten up inside with needing them--and they simply don't need you. That's all there is to it, and neither of you can do anything about it. And they'll be the same way about someone else, and someone else will be the same way about you and it goes on and on--this desperate need--and only once in a rare million do the same two people need eachother."