There is a heridatary condition called androgenetic alopecia which may affect about 50% of women. Although it mostly occurs in the late 50s or 60s, it can happen at any time, even during teenage years, Typically, each time a normal hair follicle is shed, it is replaced by hair that is equal in size. But in women with female-pattern hair loss, the new hair is finer and thinner. The hair follicles are shrinking and eventually they stop growing altogether.
Other reasons for hair loss include extreme stress; physical trauma like surgery or intense illness; dramatic weight loss over a short period of time; and taking too much Vitamin A, and hair loss can occur a couple of weeks to six months after any of these experiences.
One other way to thin hair is self-inflicted - hairstyles like cornrows or too-tight braids can cause hair loss called traction alopecia. If you get your hair done and your scalp is hurting, chances are you are damaging your hair follicles. If you keep your hair pulled too tightly, you may lose your hairline permanently.
All of the things women do to manipulate their hair -- dyes, chemical treatments, relaxers, bad brushes, blow dryers, and flat irons -- can result in damage and breakage. This includes brushing too much and towel drying aggressively when the hair is wet.
Luckily, for most of these issues, the hair grows back or the loss can be reversed with medical treatments. Most need to be prescribed by a doctor, so it is important to see a dermatologist if there seems to be something wrong, because the sooner treatment is started, the better the chances are for improving your growing season.
(Adapted from WebMD)