There are risk factors for breast cancer.  The ones you cannot control are getting your period early in life, your age, and a family history of breast cancer.  The risk factors you can control are hormone replacement therapy, not smoking and not using alcohol excessively.

Left out of the statement above is the most obvious of risk factors -- gender.  Men do get breast cancer, but the likelihood of women getting breast cancer compared to men is 100 to 1.  That said, no one knows the exact causes of breast cancer and doctors cannot really explain why one woman gets breast cancer and another does not.  
 
Risk factors you can't control

Getting your period early in life is one of the big three uncontrollable risk factors and it all boils down to estrogen.  The longer a woman is exposed to estrogen (whether it is made by the body, taken as a drug or delivered by a patch), the more likely she is to develop breast cancer. Women who began menstruation at an early age (before age 12), went through menopause late (after age 55), never had children or who took hormone replacement therapy for an extended period of time are at an increased risk for developing breast cancer.

Age is by far the greatest risk factor. Breast cancer is extremely rare under the age of 30, and is unusual up to 40. The risk of getting breast cancer goes up to about 18% among women in their 40's, while about 77% of women with breast cancer are older than 50 when they are diagnosed. 

A family history of breast cancer is a predictor for about 5% to 10% of breast cancers.  Having one first-degree relative (mother, sister, or daughter) with breast cancer approximately doubles a woman's risk, and having two first-degree relatives increases her risk five-fold. Risk is also higher among women whose close blood relatives have this disease. Blood relatives can be from either the mother's or father's side of the family.   The National Cancer Institute states that risk is higher if the relative(s) developed breast cancer in both breasts, or developed breast cancer before menopause. Also, a family history of ovarian cancer, increases the risk of breast cancer.

Risk factors you can control

Hormone replacement therapy was once thought to improve a woman's odds of not getting breast cancer.  However, in 1999, a study in The Journal of the American Medical Association (JAMA) linked hormone replacement therapy (HRT) after menopause to increased risk of uncommon forms of breast cancer that have good prognoses. HRT was not associated with the more common types of breast cancer, invasive lobular or ductal breast cancer and ductal carcinoma in situ (DCIS).

There is currently research being done to determine the link between cigarette smoking and breast cancer, and we do know that smoking affects overall health and increases the risk for many other cancers, as well as heart disease.

Use of alcohol is clearly linked to increased risk of developing breast cancer. Women who have 2 to 5 drinks daily, have about 1.5 times the risk of women who drink no alcohol. Alcohol is also known to increase the risk of developing cancers of the mouth, throat, and esophagus.

Remember that all risk factors are based on probabilities, and even someone without any risk factors can still get breast cancer. Proper screening and early detection are the best weapons in reducing the mortality associated with this disease.