Hair Loss in men has been rampant in recent years. Losing hair is a natural activity and about 85% of hair is active and in the growing phase (anagen) and the rest 15% is in the resting phase (telogen). Thus you tend to lose about 100 hairs every day and that is quite normal. And this also shows that hair loss is actually quite paradoxical, you lose hair to grow more. But there are times when you will lose more hair and you will also seem to notice that you lose hair in patches and also your hair starts to grow thin. But even this can be explained when it happens within a short time, it may be because you had a fever or else there was an infection which caused the temporary shock and your body will recover within 3 or 4 weeks after the cause subsides.
The most common cause of hair fall in men is genetic. If you notice that your parents or your relatives lose hair, then it is quite plausible that you would too and which could develop into Androgenetic Alopecia (AGA).
Cicatricial (or scarring) Alopecia can also be a probable cause. In this case, your immune system starts attacking hair follicles causing them to shrink and turn dormant, thus the production of new hair doesn't happen. If this happens then immunosuppressants are used. But other techniques like minoxidil and mesotherapy have also shown good results.
But if none of these works then for men it is best to undergo a Hair Transplant. This technique boosts hair growth and also takes hair from your own scalp which is stronger (for that reason scaling is done) and then they are removed from that area (called the donor area) and transplanted into areas that require hair (recipient area).