Greasy skin stems from excessive sebum production by your sebaceous glands. But before you start hating on your oily complexion, sebum has its benefits. It maintains your skin’s natural moisture barriers and protects it from bacterial and fungal infections. However, excessive sebum will clog pores which will result in problems like bacteria growth, acne, whiteheads and blackheads. Here are nine reasons why your sebaceous glands go into overdrive.