A person's age, genetic factors, personal health history and diet all contribute to breast cancer. History of previous cancer in one breast, especially if it occurred before menopause increases a woman's risk of developing a new breast cancer unrelated to the first one. About 5% to 10% of all breast cancers may be related to genes that are passed through close relatives such as mother or sister, who developed breast cancer before menopause. Other factors include

First child over age of 30, hormonal changes during the time of pregnancy may influence a woman’s chances of developing breast cancer.
Not having children
Early menarchy (before age 12)
Late menopause
Over weight especially in the upper body
Use of alcohol
Excessive radiation
Hormonal contraceptives.
Hormone replacement therapy for more than five years.
Usage of diethylstilbestrol to prevent miscarriage.
Ovarian cancer patients.