(This is probably a naive question, but...) why is love so incomprehensible and impredictable?

I think this is because everything that happens to us in life changes who we are, day by day. And although we may be with someone today and feel they are the most amazing person and we were "made for each other" something could happen tomorrow to change either one of us and in doing so it changes our compatibility in some cases also the way we feel about people and things.