When it's not because they're the most gorgeous person, or the smartest, or the richest. But you just like listening to them, and are romantically attracted to them.
You text them, share pictures of each other, and share similar interests.
When they're a mystery to you and when they let you into their world in small bits, you are amazed and fascinated.
When you could potentially kill what you have with this person because you feel this way, knowing there's a high chance they dont feel the same for you, but they could and they would never show it.