What could cause someone's breathing to be more audible than most people's?

I’ve noticed this for years (ever since I’ve known him; way back before he was my boyfriend) in that my boyfriend’s breathing is more audible than most people’s. I had always wondered about it and when I realized it was something that happened all the time, I knew it wasn’t just from a cold. I was too embarrassed to ask him about it, but finally someone mentioned it to him and so he mentioned it to me and said that he had been tested for asthma and such (at a young age), but didn’t have it. It’s not something he notices—it’s just always been that way.

What would cause something like that and is it dangerous? I’ve noticed that after exertion (like after we came back from biking one time) his breathing was even more loud. He doesn’t snore or anything, for the record. He just breathes more loudly than most people.

It was kind of funny when he finally mentioned it, saying that a girl told him he breathes too loud and I said “oh yeah, Rory, I’ve noticed that too, for like the past six years…”. I just didn’t feel comfortable ever mentioning it!