I never said it was the most prevalent

Though it is a common mental illness or disorder BDD does affect a lot of people and often does stem from Depression, or vice versa depression can stem from the disorder. Like many illnesses a base can be depression itself. Many people have to seek therapy in order to correctly diagnose the reasons for having the disorder.
And yes like above you can also have OCD which causes you to inadvertanly choose to do things or feel certain ways about things without wanting to... I have heard of that. Seen a special about it on Discovery actually last month.