On average, most people lose 50 to 100 hairs per day which isn't generally an issue as new hair is constantly growing to replace it. Where hair loss truly becomes a concern is when the loss begins to exceed the natural natural replacement or when the hair follicle is damaged or scarred.
When hair is lost you should also take time to examine the hair itself to see if it has been broken or has been shed at the root. Understanding this will help guide you through restoring or prolonging your hair.
Hereditary, Genetic (inherited from family): Male and female pattern baldness can begin as early as puberty, trademarked with receding hairline and bald-spots in men and thinning hair in women. In general, topical creams or oral medications are the best path forward.
Stress or Shock: Both physical and mental/emotional stresses have been linked to hair loss, occurring weeks to months after a traumatic or stressful period. Thankfully it is most often times temporary.
Lack of Protein or Iron: Diets lacking in protein or iron may dramatically slow hair growth and lead to thinning and weakening of hair. This loss is reversible through improved diet or supplements.
Harsh Products or Treatments: Harsh shampoo, styling products, chemical relaxers, permanents, hair dyes or bleach can irritate the scalp causing inflammation of hair follicles leading to hair loss. This hair loss can be permanent if the scalp is damaged or scarred.
Hairstyles: Certain hairstyles such as tight braids, pigtails or cornrows can cause "traction alopecia" which can be permanent depending on the damage to the scalp.