Hair loss is never a pleasant experience. It can be caused by many things, such as a condition/illness, or something that you genetically inherited. It can be caused by the lifestyle you're leading, chemicals you are using on your hair, and the way you brush it. It might be a medication you're using, or maybe a treatment such as chemotherapy. Or simply, a head injury. Most of these are completely reversible, except for the hereditary type.
If you know, or you think you know, you're not sure, or you absolutely have no clue why this is happening to you, and you want to find a solution, this is the page for you. However, it is strongly recommended that you visit the doctor anyway, before starting any treatment by yourself.