Why does he do this?

When my bf and I are messing around and I try to go down on him he’ll usually try to stop me and he’ll go down on me instead even if I ask if I can do him. Even when I do start the bj he’ll still try to play with me which will make it hard for me to do my part or he’ll put me on his face so we’re 69ing. I’m just wondering what would be the reason why he does this? I’m starting to think he doesn’t like my bjs or something but when I do them he’s usually really into it and moans occasionally soo i’m confused. Any ideas?