Hair grows in response to hormones and individual biochemistry. Heredity, genetics, racial background, medications, emotional stress and sometimes diet can effect hormone production. Things such as shaving, tweezing or waxing do not cause hair to grow.
Hair grows in cycles.
The first part of the cycle is when the cells divide and produce more hair or grow. During this phase, the follicle pushes down into the skin called the dermis. This is called the anagen or growing phase. Once the growth has occurred, the cells stop dividing and the follicle begins to break down and move back up to where it started. This is called the catagen or shedding phase.