Infatuation can happen at any time. It's a sudden rush of attraction; an immediate connection to someone, even if you don't even know their name. It may pass as easily as it came, but other times, that nagging feeling of being "in love" with someone lasts for a while.
As far as evolution goes, whoever reproduces the most, wins. In order to give offspring their best chance at survival, animals need to make sure that their mate has good genetic qualities that are worth passing along. Having several mates over a lifetime makes your children genetically diverse, increasing the odds that they'll grow up and have children of their own.
We can become attracted in a moment because of subconscious traits we see in the other person that make us want to mate with them. Far from being a sweet little crush, this instinct is telling us to go after that person and start to get busy.
Of course, that's not really how humans decide to raise a family anymore. This kind of behavior really doesn't work in our modern societies, so it's likely a leftover evolutionary trait that we still have but don't need, like wisdom teeth.
Want to learn more? Listen to Bill explain it here:
Not as romantic an explanation as you would have hoped? Evolution can be a cold mistress.
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