Why does shaving cause folliculitis?

In grown hair: Hair can curl back during shaving and go sideways into the skin. This may be infected and cause folliculitis. The only way to prevent this is to stop shaving or use tweezers.

Ingrown hair: Folliculitis centers around a follicle in which the pore is clogged with debris or in the case of shaving, a hair that starts to grow inward on iteself. The latter also clogs the pore, resulting in oil and dirt build up, bacterial growth, inflammation and pus formation.