The reasons are highly individual, but can include any combination of the following:
Stress (emotional and physical)
Hormonal imbalance, specifically androgen sensitivity
Genetics
Immune system irregularities
Thyroid disorders
Nutritional deficiencies (especially iron or vitamin A)
Cosmetics (allergies and harsh treatments)
Radiation/chemotherapy
Dental treatment
Blood loss
Drugs
Disease
Surgery
Scarring