Pimples occur due to hormonal changes that take place during puberty/adolescence. During puberty/adolescence, the activity of sebaceous (oil) glands becomes high. This leads to an increase in the oil production by the skin, which leads to microbial growth on the skin. This microbial growth is the reason for pimples. In order to reduce the growth of microbes, it is essential to keep the skin clean. Thus, the adolescents who do not keep their skin clean experience pimples.