I don't really fit into any of the above it started when I was eleven and I've still got it. I think Hormones may play a part. There is very little known history of acne in my family my father had it as a tenager but grew out of it in his early twenties.
I don't know why

Well even my father grew out of his acne by the time he was 22-23. But he has always had oily skin and seborrheic dermatitis. Even SD is due to oily skin. And he has HUGE pores, its literally like holes on his face. He is allergic to a lot of things. I am thinking of doing an allergy test for myself as I feel my oily skin is a result of being allergic as well. But if I find I am allergic to dust or something, there is nothing I can do about it. Right now my acne is under control with antibiotics. If and when they stop working, I will have to go an accutane...
But I am also hoping that after pregnancy, my acne will go away. I have heard it happens for some women. Also my pores may become huge that acne can't form anymore. Already my pores are huge at 30 but maybe not big enough for acne not to form. I know most women want to minimize their pores, but I don't really care. Truth is I am really scared of accutane and its side effects...