What does it mean to have good sexual chemistry?

My boyfriend and I have this really good connection and while it’s mostly physical, it seems to be more than that but I just can’t explain what it is. Before we started dating I asked him why he liked sleeping with me and he said that anytime he was with someone else things just didn’t feel as good and he said it just didn’t feel right because they weren’t me. He also said the other day that he’s been addicted since we met. I’ve felt that too like anytime I was with someone else I just wasn’t able to get into it and it never felt as good. What would you call this?