It's a recognition of some one from your future. Either an intense and wonderful but brief acquaintance that'll be fondly recalled always, or a full long-term r/ship, or even unrequited love where all you do is think and focus on this person for a long time. All are large mental territories that, if the future can be read, maybe love at first sight is just spotting the future!
Consider where these ideas originated. I think from combined human experience thru the millenniums. But these cliches are far too generalised; it's only certain types of redheads that are on fire - some are timid. Line me up 50 natural blonde women (I don't know about men, I've only really studied women) and I reckon I can rank them by IQ just from looking at them, and score significantly. Bet that you could too.
I'll even contend that beautiful people are beautiful inside, and that ugly folk have less personality. But I can't program a computer to what beautiful looks like, and even my best mate will have a different opinion.
I've travelled for many years, worked too many jobs, and have known many many people. To the point where I meet someone who has uncanny resemblance to a prior acquaintance. And guess what - always they have similar personality. So if I meet Anne, who looks like old pal Barbara, I'll think of her as a Babs-alike, and have the social advantage of already knowing her quite well. I think I'm the only one who does this, and I don't expect others to follow, and
Just think of stereo-typical looks, and ask yourself where the stereo-type came from, then ask yourself if looks matter. And remember that there are exceptions to every rule, and rules to those exceptions.
Better example, more realistic: A vicious dog could still attack you if you simply ignore it as a method of survival. But if you were an alien, that not only didn't know a dog was dangerous, but actually was not interested at all, you would not be harmed. Curious alien gets eaten.
Everyone does this. See a pebble, kick it. But why? I believe that pebbles
are alive, but only slightly. Occasionally a pebble (rock, stone etc) gathers
enough strength to send out a wish to be kicked, so it can hangout somewhere
new for a while. Us humans hear that wish, we kick, the pebble smiles inside.
However, I doubt that flies bug us because of a death wish they have.