1. He’s embarrassed by them.
Everyone has that friend they’re only still friends with because they’ve been friends since birth. It’s their only common denominator at this point because they’ve grown into such different people. Maybe that’s all of his friends, and he knows you wouldn’t be able to stand five minutes in a room with them. Maybe his best friends are a monkey that wears a vest and a magic carpet, and he’s pretending to be a sultan.
2. He acts differently around them.
He might think you’re not going to like the way he is when he’s with them. Maybe they bring out the binge-drinking idiot in him, or the giant nerd, or the guy who can only speak in South Park references.
3. They’re dicks.
He knows he’d be throwing you to the hyenas like you’re Scar at the end of The Lion King if he brings you around his friends. He’s trying to protect you because he knows they’re super judmental.
4. He’s not that close with them.
Some people don’t really surround themselves with close friends, just drinking buddies or casual acquaintances. He might just not have anyone that he feels is important enough to meet you and vice versa.
5. They're party animals.
Maybe his friends are so YOLO-CHUG-THIS-BEER-BONG-HOW-MUCH-WEED-CAN-YOU-FILL-YOUR-LUNGS-WITH-AT-ONCE intense that he’s afraid of bringing you around and scaring you off forever. If every one of his Friday nights could be a scene from The Hangover or The Hangover Part 2 (NOT part 3), then he might not want to expose you to that world.
6. They don’t live nearby.
Some people stay great friends with their high school buddies, and after everyone goes to school and gets jobs and shuffles around the world, meeting up with them for an introduction might not be that feasible. Don't freak out if he hasn't arranged for a visit by three months. You'll meet them eventually.
7. He thinks you don’t want to meet them.
He might just assume you have no interest in meeting his friends. It never hurts to tell him, “He sounds fun. When can we all hang out?” Communication!
8. He doesn’t think you’re that serious.
Meeting his friends is like meeting his parents: He’s declaring that he plans on you being in his life for a while. If you guys are still in the casual dating phase, or he thinks you are, he's probably not going to bring you around just yet.
9. He hasn’t met your friends, either. Even though they're So nice.
This is a friends-meeting stalemate. If he hasn’t met your friends, he probably doesn’t want to introduce you to his. The answer to this problem is: Plan a mixer. Invite everyone out to a bar and have all your friends meet and all become best friends and you can live in apartments across the hall from each other/Joey and Chandler.