Why doesn't my boyfriend, who is 22, want to be intimate with me, a 21-year-old woman?

Why has my boyfriend of five years suddenly lost interest in sex? We used to be intimate about four times a week, and sometimes even twice a day. Now, it’s down to once or twice a week, and recently even just once a week. Last Tuesday, we had sex twice, and since then, nothing. He used to be the one to initiate things, but that hasn’t happened at all recently. Last Saturday, after a night out, I asked him if we could be intimate, but he claimed he was too drunk and tired. I accepted that and didn’t bring it up again. It’s now been nearly a week of hanging out together, and he still hasn’t made any moves. I don’t mean to sound boastful, but I know I’m attractive; I get attention from other men often. When we are intimate, he always comments on how great it feels and questions why we don’t do it more often, yet he seems to completely forget these moments and doesn’t want to engage more frequently. It doesn’t make sense to me. What’s going on? **TL;DR:** My boyfriend of five years has drastically reduced our sex life, despite expressing that he enjoys it. Why?