About half of girls aged 14 to 17 years (and roughly three-quarters of boys) have pimples. Generally, upon reaching 20 the problem has dissipated and only appears a few others from time to time.
Acne is not a result of a skin “dirty”, poor hygiene, sweets, the imminence of an important appointment or greasy hair on the face. There are real causes of acne, all related to the increase in androgens (male hormones) around puberty. Some people have hormonal imbalances but they do not pimples (which is extremely offensive to others). And there are people whose skin just seems more likely to react badly to androgens.
Another reason that androgens may cause pimples is that increased levels of the hormone in the skin may cause a thickening around the pore and an increase in sebum (oil). The bacteria that grow on the surface of the skin can infect the pores and cause inflammation. This seems to be genetic, so when in doubt, blame your parents, by now they should be used.