When you fall for someone, your brain releases a cocktail of happy chemicals, including oxytocin, the cuddle hormone. The feeling is magnified if you have sex with that person.
A nice thing about getting older is you can separate the butterflies, infatuation feeling from someone’s actual merits. Having the butterflies for someone makes you prone to idealize them and explain away their flaws (“I just am down to be with them and work around our differences”). It’s important to mentally step aside for a second and think about who they really are. What if your friend or sister had a crush on them? Or what if you had a daughter, and she were crushing on someone like them? Would you want them to enter that relationship?
I’m guessing (since I’ve been there) that you fall for these people so quickly because you daydream about them a lot. Or maybe you imagine things about them that you haven’t learned yet (like, he seems smart, maybe we share the same taste in books, he’s special, he gets me.) And if you are young or have low self esteem, it’s really easy to let your imagination run away over stuff like that. Because if you have low self esteem, it’s easy to think that other people don’t really care about you that much, which makes the crush seem that much more special. (Spending quality time with friends and family is a great antidote to a runaway crush.)
Daydreaming is like playing a tape in your head where nice things happen, it makes you happy, and it almost certainly releases all those happy crush-making chemicals in your brain. I’m not saying don’t daydream, because that’s half the fun of having a crush! But you should be aware of how it might affect you. It puts more into a crush than what’s really there.
With experience, you can be an objective judge of a person’s character while still having a massive crush on them. You can even kind of enjoy it. It’s a pretty special feeling, after all.