Hi Jane, I am pretty sure the only reason is because there is a made up sentence "love at first sight". and they use that in this sentence. Javier

It's a set phrase, so there's no the. Adding another word makes the sentence longer and thus more convoluted by reducing the flow. "Love at first sight" is simply the phrase that is always used. You'd never say "It was just love at the first sight" in 99% of cases.

i don´t think is wrong...It just does not sound right. love at first sight sounds better.