I’d guess that it ultimately boils down to a fear of social rejection and a fear of missing out on an opportunity to mate.
Our brains haven’t evolved to keep up with modern society. Back when our brains evolved, social rejection meant we’d be dead. Literally death. That’s why embarrassment, shame, and fear of rejection can be so powerful. They could mean death.
As well, we could die at any time. So missing out on a chance for our genes to be passed down to the next generation? Well, we can’t die before that happens! We need to make this crush work NOW so we can turn it into a relationship, or at least a mating session, and pass on those genes!