Well, first of all, let's not forget that he is in a relationship and he will probably have a good reason for it. But let's analyze the question a bit more closely.He Likes You!
This is always the first possible answer. He simply likes you and he is embarrassed or afraid to talk about his girlfriend in front of you.
He probably thinks that if he mentions his girlfriend in your conversations, he will never be able to make you know he is interested in you.
He could also be afraid that you might get the impression that he is just friends with you and that there is no chance of developing any other kind of relationship.
But how can he be so interested in you and still love another woman? Well, he might have just recently discovered that he doesn't have strong feelings for her anymore, and now he is trying to end the relationship in the least painful way possible and without hurting her.
Consider that it isn't easy for him either. You have to give him time if you like him.
He thinks that you like him, but he loves his girlfriend and won't talk about her in front of you to because he is afraid that this might hurt your feelings.
This can be a very embarrassing and difficult situation for him. He sees you as a friend and doesn't want to lose you, but he doesn't share your feelings either.
He's Just Flirting
When a guy just wants to flirt with you and have fun, he will never talk about his girlfriend in front of you. Don't expect to develop a relationship with this type of guy.
He already has the life he wants: he has a girlfriend that he probably loves and he just wants to flirt with other girls for fun. He is actually leading two separate lives. You will never be part of the other one.
In this case, you can either decide to accept his game and have fun flirting, or show him that you are not interested at all.
He's Not in a Serious Relationship
He doesn't have a serious relationship with his girlfriend and perhaps he is going out with other girls too.
He doesn't talk about her because she is not important to him and they see each other only once in a while. Well, he still hasn't found what he is looking for.
If you like him, go out with him and try to get to know him better. You never know what life brings. Perhaps he will discover that you are the right one!
I hope these tips have helped you better understand why guys won't talk about their girlfriends sometimes. If yes, share them!