The causes of hair loss are plenty including diseases,
nutritional deficiencies, hormone imbalances, and even
stress. But, one of the most common ones is adrogenetic
alopecia.

Alopecia, is in fact, the term for hair loss. Androgenetic
is a reference to the fact that factors such as a genetic
predisposition to balding and the influence of male
hormones-androgens-play a part. There is also a third
aspect-aging.
Let's look at these in brief:
Genetics
Genetics is a complicated thing and not easy to understand
or predict the behavior of. The case with balding is also
the same. Simply the presence (or absence) of balding in
one's parents, on either, the mother's or father's side, is
not necessarily predictive of one's likelihood of balding.
It's very hard to accurately predict who will go bald and
how rapidly.
Androgenic Hormones
All normal men and women produce "male" hormones. The most
common of these are testosterone, androsteinedione, and
dihydrotestosterone (DHT). These hormones are quite
important in both sexes, but occur in different
concentrations, being much more predominant in males than
in females.
This, in part, is responsible for the typical differences
between the genders. It is the exposure of the hair
follicles to DHT, in a genetically susceptible person, over
a period of time, which leads to androgenetic alopecia, or
male and female pattern baldness.
Aging
Age and balding are not proportional in any manner
imaginable. It is a process and this fact should not be
ignored. Like any process, it can be rapid or slow, it can
begin toward the end of life or in the late teens, and it
can progress in a predictably inexorable fashion, or it can
stop and start, seemingly stabilize, and then begin again.
But what exactly does happen? Assuming we have a
genetically predisposed person, then as the follicles are
continuously exposed to DHT, an interesting phenomenon
occurs. The anagen phase, or active growth phase of the
hair becomes gradually briefer and briefer, and eventually
the hair becomes finer and shorter, and less deeply
colored.
This is called "miniaturization" of hairs. This is also the
point at which hair loss tends to be noticed first. It's
not that there are fewer hairs on the head, but that their
caliber (cross-sectional area), color and length are so
diminished that they no longer provide "coverage" for the
scalp beneath.