There are an array of factors that 'promote' hair-loss. Any form of harsh hair therapies, chemical products, hair dyes and hair straightening procedures (if used regularly) can lead to hair-loss as the hair follicles get damaged. Hair-loss can also be hereditary - most commonly in men. Hormone fluctuations may lead to hair-loss however doctors usually prescribe the contraceptive pill to counterbalance the hormone imbalance. Pregnancy (due to hormone fluctuations) can also lead to hair-loss as hair is retained on scalp and only falls 3 months after birth. Another common hair-loss enforcer is a poor diet lacking vital nutrients and vitamins as well as anorexia and bulimia. Finally, hair-loss may just be a natural result from aging whereby the hair-growth process naturally slows down.
Whatever the exact root of your problem the best thing to do is to visit your dermatologist who will help you override this common problem.