The one where you're just getting to know them and you think they're cute. You have some common interests and they're nice to you.
Then there's the serious kind, the one where you're put in the situation where you have to make decisions, they want you. You have potential to be happy with them. You know their flaws, yet you don't care. You fall hard and fast and it's not just the present you're thinking about anymore, it's the future.