I really hope this question wasn’t prompted by dudes always sniffing your hair, because if so I’m going to be involuntarily shuddering all week, and my advice is to carry a stun baton around and MY GOD HE’S RIGHT BEHIND YOU!
But I’ll protect my own sanity by assuming you’ve seen this a lot in movies and on TV and that’s why you asked. And yeah, movie dudes seem to love the smell of hair, that’s true. I think it’s more of an old screenwriter’s trick to show “he’s into that girl.” Sort of like how to show someone’s depressed in a movie, they always pan to an old framed photo of a boot. We know that’s not realistic, but we all “get it.”
Don’t get me wrong; chick hair smells fantastic most of the time. And there’s no question men like the smelly oils you’ve got all up in there. But I doubt most guys would actually go for a hair sniff if they weren’t already in bed with you, married to you, or had some other equally believable cover story.
So the short answer is: because it smells good. The long answer is that guys don’t really do that very often in real life, unless I’ve been attending the wrong orgies.