Sometimes people have different needs for physical affection. Sometimes people change physically. Maybe they develop bad breath. Sometimes… sometimes…. I can’t begin to think of all the possibilities.
The bottom line is you have to talk about it and find out from him what is going on. If it’s bad breath, he may not want to say because he might feel embarrassed to bring it up and to make you feel bad. I really have no idea. But he’s the only who knows and you’ve got to talk to him about it.
When you talk to him, try not to put him on the defensive. Ask him what is going on. Never accuse him of something. Sometimes, even a question can be an accusation. “How come you don’t kiss me any more?” Sounds like an accusation and probably will get a defensive reaction. “What are you talking about, girl? I kiss you all the time!”
So something like, “Boyfriend, you know I love kissing. Lately, I feel like I want to kiss more. Could we do that? Is there any reason why you might not feel like kissing me so much?” You want to say it in a way that allows him to speak up, and you also want to shut up so he can say what he wants to say. Don’t get defensive about it. You really want this information so you can fix the problem. It doesn’t help to deny a problem.