What is love?

What is love? I kind of regard it as being something like a total and complete appreciation of another person's essence, a total empathetic identification with their being. Often people say love is all in the mind, or it is merely chemicals. But what would it mean to say that such feelings are in one's mind? How would this differ if they were not in one's mind? Is mere liking of a person also in one's mind? Hating a person? Admiring a person?

I would guess they mean that love is caused by processes occurring in the brain. But how do we know the causal relationship always runs from the brain to the mind? Perhaps feelings of love precipitate processes in the brain?