Im soo tired of the trope of people falling in love instantly. Thats not how it works. Im not gonna care about the story with no proper build up.
Actually there were some studies done on attraction and it was found that Humans tend to figure it out within the first 3 or so seconds whether they're mildly, intensely or not attracted to a person at all.
Love... Love is an ephemeral concept that can't really be pinned down. But compatibility... Our cross-wired little monkey brains tend to figure out whether we 'think' we're in love pretty easy too. Add to that, the fact that our social norms and taboos are informed by the media of the era... Well of course we fall in love at first sight. We're conditioned to think all the biological responses we have upon looking at someone equate to love, hate or indifference.