According to Wikipedia, hair loss has the following causes:
Male pattern baldness
Fungal infection such as "black dot" tinea or tinea capitis
Chemical breakage such as over processing, or frequent use of chemical relaxer
Heat damage as from repeated hot comb use
Chronic exposure to traction on hair shaft such as Traction alopecia
Compulsive hair pulling such as trichotillomania
Telogen effluvium resulting from physical or psychological stress
Hereditary disorder of the hair shaft or genodermatoses
Secondary syphillis can cause "moth eaten hairloss"
Discoid lupus erythematosus or chronic cutanous lupus erythematosus
Pseudopelade of Brocq
Keratosis follicularis spinulosa decalvans
Adverse effect from certain drugs such as chemotherapy
Testosterone booster tablets.
Question: Do you exercise and eat well (balanced diet)? Anorexia, bulimia, and other eating disorders, as well as a sedentary lifestyle and unbalanced diet can contribute.
The most common reasons are: lifestyle (eating too little or too much), genetics, hormone imbalances, stress, hypothyroidism....