The reasons or factors that cause breast cancer are:

Early menarche & late menopause
Older age at first birth or never having given birth
No breastfeeding
Family History of Breast cancer
You can go through this article to understand all other causes: Breast Cancer Types & Risk Factors

And the prevention methods are:

Keep Weight in Check
Avoid Birth Control Pills, Particularly After Age 35 or If You Smoke
Avoid Post-Menopausal Hormones
Donít Smoke
Be Physically Active