One of the least pleasant things about being human is the appearance at adolescence of pimples. Not only do they make their entrance just at the time when people are most self-conscious, but they come and go for the rest of your life. It would almost be better to have a hairy face.
Ironically, the general covering of our bodies in hair - what we call "fur" on other animals - is at the root of our problem with acne. In the course of evolution human beings lost their entirely hairy faces and now have a lot of exposed skin there: even a bearded man with bushy eyebrows has much of his face bare.
The structures in the facial skin that once gave rise to hair, however, have not disappeared entirely: many still contain tiny vestigial hairs which do not grow. That means there is nothing to open up the pore itself; and that means the pore is a great place for oils, dirt and bacteria to get clogged. You can get zits anywhere vestigial follicles become clogged.
The big culprit on faces, though, isn't dirt so much as an internally-produced substance called sebum that makes the skin cells shed inside the pores. These dead cells, unable to flake away, collect until they become an irritation to the pore. At this point the body reacts in a miniature version of the way it deals with any other invasion: it sends white blood cells to the site to attack the invading body. The result is a mild inflammation at the scene of battle. That inflammation is the zit.
So evolution, while having cleaned our hairy faces, gave us pimply skin instead.