There are different reasons for hair fall. In women, it can be because of pregnancy, stress, hormonal changes, and Polycystic Ovarian syndrome (PCOS). In men, it is mainly because of Androgenetic Alopecia, which is basically increased sensitivity of the hair follicles to the male hormone Dihydrotestosterone (DHT). Other reasons for hair fall include lack of nutrients, ageing, medications, thyroid diseases, anaemia, chemotherapy, vitamin and protein deficiency, secondary syphilis, and scalp infections. Even hair styling, chemical treatments to the hair or tying your hair too tightly can cause hair fall. Dr Rinky Kapoor, talks about in detail about reasons of hair fall at https://www.youtube.com/watch?v=s_4JXVxdFp4