Breast cancer risk is highest for those who are in industrialized countries. The large majority of breast cancers are lifestyle related.

High calorie intake leads to early puberty and breast development.
Delaying child birth for education and career lead to many extra years of estrogen
stimulation of immature breast tissue.
When women have babies and nurse for a long time, that estrogen stimulation is stopped and the breast feeding results in maturation of breast tissue.
This factor is a very important risk factor and may account for about one third of the
higher breast cancer risk in developed countries.

Obesity and lack of exercise contribute to another one third (or more) of the risk.
This is because obesity and lack of exercise result in higher blood sugars and higher
insulin and estrogen levels.
Insulin is an very effective cell growth promotor.
Estrogen stimulates breast tissue.

Many women are under the impression that they can't get breast cancer because they
don't have anyone in their family who has had it.
That's a very wrong misconception.
85% of women diagnosed with breast cancer do not have a first degree relative with
breast cancer.
Only 5-6% of breast cancer is due to an inherited breast cancer gene like BRCA.

There is some data to suggest that alcohol contributes to risk as does smoking, but to a
much smaller degree than the other risk factors.

More Info... https://www.quora.com/What-are-the-major-causes-of-breast-cancer?no_redirect=1

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