Change of season, diet or medication can all cause you to lose more hair. Also if you're a woman who has recently given birth you will lose a lot of hair in most cases. Then there's the hereditary reasons. If balding is in your family then you should probable see a Trichologist (a doctor who specialises in hair and scalp) to see if there are any solutions for your particular problem.
If you're still losing hair and can't relate back to any of the above reasons then here is the last and most common reason for hair loss. Stress. Long term stress and anxiety will cause you to lose a lot of hair and you'll notice when you calm your nerves for a long period of time there will be a lot of short hair growing at once from your roots. This looks dreadful and makes styling your hair that much harder. So avoid it before its too late.
Relax, do some meditation or let that stress out whatever way works best for you. You'll thank yourself for your thick healthy hair