[Suvash]

well, i am not actually a specialist but as far as i know, pimples are a result of a blockage in the ducts of numerous glands in our skin, they are especially numerous in the face. some ppl have oily skin and have a large number of glands so if they could not take proper care of their skin to keep the pores open then they might get pimples due to the collection of the secretions.
and stress may also increase the pimples so friends relax stay stress free.

and lets wait and see what others have to say.