Calling Him Daddy

So whenever I have a specific guy I "talk" to, he usually ends up asking me to call him daddy(not during sex or anything I don't do that stuff).But like just in general over text, FaceTime or face to face. Why do guys like that. The guy I'm talking to now says it's because I'm smaller and it makes him feel dominant or something. Does anyone have any other reasons? And this might be hard to answer considering this is a girls app.