What causes the dimples in someone's cheeks? What about dimples on top of the butt?

Can't say about the. Can't say there is a cause other than inheritance for dimples of the jaw of cheeks. However, the dimples above the pelvis are anatomically called the dimples of venus, and represent the space between 5th vertebra and the pelvis. They are more visible in thinner people and can be obscured by adiposity!