Because he’s a loser who believes in The One. Loljk.
Well, seriously, I don’t believe there is such a thing. There are so many people in this world that you’re bound to make a connection to some of them.
Anyway, to answer the question, this guy is most likely idealistic as hell and is looking for a Manic Pixie Dream Girl. He romanticizes the fact that they made an instant connection and doesn’t look further than that.
I don’t believe you can fall for someone quickly. Love is such a loaded word and you need to know the other person first before you can truly say you love them. You need to see them in a good mood, in a bad mood, hungry, tired, and sick before you can say you love them. Because if you can’t accept them through all their shit, is that really love?