This answer was on quora.com. I couldn't say it better, so I'll post it as a quote and give credit with a link. It deserves to be read often, and the writer, Marcus Geduld, deserves to never be plagiarized.
"When you have a crush, you are romantically fascinated or obsessed with another person, but he or she still seems to be at a remove from you. When you're in love, your identity merges with someone else's. You'd take a bullet for him or her.
When I say that your identity merges with the other person, I don't mean he or she is all you think about. You can obsess about someone without becoming "one" with them. Love means you think of yourself as an "us" rather than an "I."
You want to make the other person happy, not because you're hoping for something in return; because your happiness and the other person's happiness are one and the same thing.
If the person your crushing on dumps you or disdains you, it feels terrible because rejection feels terrible; if the person you love rejects you, it feels terrible because amputation feels terrible."