What is it like to fall out of love with someone?

I think I'm going through that at the moment, and it's pretty soul destroying. Once I thought I wanted him more than anything in the world - it felt like I was drowning in him. I was so infatuated.
Now I feel nothing. It's triggering a whole existential crisis type thing. Is it because now I know he loves me? What - now I have him I don't want him anymore? Am I that fickle and vain? Or have I just gotten to know him better, and I've realized we aren't compatible at all...?