1. You feel like you owe it to the person. "I mean, they've been so nice to me. I should just do it with them. It's the right thing to do!" Uh, no. You owe someone 13 dollars when they buy your ticket to the movies. You don't owe your vagina/penis to anyone, no matter how amazing they've been. (Unless, of course, you want to sleep with them. In which case, go for it! They sound great!)
2. You're bored. Bored sex is hell on Earth. It just amplifies the restlessness you're currently feeling and makes it 10 times worse because now you've involved another unsuspecting party. This poor fool thought they were getting invited to some potentially amazing sex but nope. You're just bored. All you can really do is moan on cue and turn over, if need be.