From what I've gleaned on the subject - and I'd be happy to learn more from anyone more knowledgeable - fairies have difficulty understanding human love, so they can't actually "fall in love" with us.

On the other hand, a human being can fall in love with a fairy, and there are many legends of human beings disappearing into the land of the fairy people.

I think fairies are more dispassionate than us, which given their work of fixing the mess humans have made of our world, is a good thing - otherwise they'd be taking the destruction of their good work for the Nature world very personally.

If anyone can add a bit to the topic, I'm sure we'd be fascinated to know more.