" Human breast cancer is a clonal disease; a single transformed cell-- the end result of a series of somatic (acquired) or germline mutations-- is able to express full malignant potential. Thus, breast cancer may exist for a long period as either a noninvasive disease or an invasive but nonmetastatic disease."

The above statement is for doctors. It more than answers your questions of why and how women get breast cancer. The details for the mechanism of breast cancer are all extrapolations of the above statement.

As a lay person, it is good to know that breast cancer is a hormone dependent disease. Therefore, women without functioning ovaries and who have never taken estrogen replacement pills will never develop breast cancer.

The role of diet in breast cancer etiology is controversial. While total calorie  and fat intake are associated with breast cancer risk, the exact role of fat in the diet is unproven.

The potential role of exogenous hormones in breast cancer is very important because millions of women throughout the world take oral contraceptives and hormone replacement therapy. Meta-analyses suggest that oral contraceptives cause little if any increased risk of breast cancer. In addition, oral contraceptives offer a substantial protective effect against ovarian epithelial tumors and endometrial cancers.

 Breast cancer can be inherited or acquired. Women who have genetic mutation forms less than 10% of human breast cancers. Two well known genes, BRCA1 or BRCA2 can now be sequenced readily and germline mutations detected. Patients with these mutations can be counseled appropriately. They have an 80% risk of developing breast cancer. Women who have first-degree relative diagnosed to have breast cancer increase their risk of also acquiring breast cancer. Moreover, women with first-degree relative that are diagnosed to have breast cancer before menopause increase the risk for them in acquiring breast cancer.

Some non-specific  factors increases the risk of breast cancer. You should be able to figure out how these general factors increases risk of breast cancers. These are: smoking, alcohol and radiation exposure. Women who are smoking will increase their chances to have breast cancer. High intakes of alcohol have been found to be a source of breast cancer. Radiation exposure is another factor that contributes to breast cancer. Studies have shown that women as well as children who have undergone high-dose radiation therapy have a much higher chance of having breast cancer.