Although it is true that outside factors (dirt, grime, makeup, etc.) contribute to clogged pores and bacteria which can cause acne problems, hormones are the key factor in the development of acne.
Acne breakouts begin with the negative effects of the hormone DHT(dihydrotestosterone) on the oil glands. The enzyme 5 alpha reductase converts testosterone to DHT. It then binds itself to androgen receptors on the oil glands, stimulating excessive oil production. This excess oil obstructs the skin pores allowing bacteria growth, which in turn causes inflammation, infection, and visible acne breakouts.