Well, not everyone believes in a soul mate. Some people believe that a person is just more compatible than others. It's not such a mystery
Anyway, I think he might have had a relationship in the past that made him this way. He was probably in a serious relationship and he was hurt by his partner so badly that he develop these jaded ideas about love. It could also be that his parents' divorce caused him to be think this way like Adrastus said. The best thing to do right now is just to let him be; there will be someone that will change his mind about jaded love.
His ideas about that shouldn't really affect you much, that's just the way he thinks. In general, you shouldn't be bothered by what others think about the world. You won't be able to change a person, really. You could do it, but it's really, really hard.