Let's face it. Sometimes, love makes absolutely no sense. So why does it have to in a book? I've read reviews that were negative because readers couldn't figure out why the hero and heroine fell in love. Real life is strange. But fiction isn't supposed to be. That's not my opinion. I think the stranger the story, the better. People fall in love for all sorts of reasons. But it all usually starts off with what is pleasing to the eye.
Think about your hero/heroine's first meeting. Their gazes meet. Sparks fly at a touch. Something is there that sets her/him apart from all others. That's where the 'why' comes in. What is it? There has to be something. You can't have a 'just because' here. Something brought them together.
So here's a challenge for the writers out there. In two sentences, tell me why your hero and heroine love each other.