It's hard to define but you'll know it when it's there. When the feeling gets stronger, but goes beyond the physical needs, then that's probably it.
Honestly it takes a lot of time to develop your feelings for a person even if you know him/her for a long time. It takes an even longer time to get over that person. Maybe those who regularly change their partners aren't exactly in love with them. It's not easy to develop and discard your feelings for a certain person, I haven't even gotten over my first love 5 years ago

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So if you saw a cute guy last week and want to marry him, that's probably just infatuation
