The most common cause of breast tenderness is a hormonal imbalance during the second half of a woman's monthly cycle when the breast accumulate fluid due to increased estrogen and progesterone levels. Breast tenderness can be caused by many things, however. Caffeine, interestingly enough, can cause it. Birth control pills or hormone replacement therapy (HRT) can and stress can exacerbate or cause it. It can also be an early sign of pregnancy or menopause.
If breast pain does not appear to be cyclic in nature and becomes chronic then you should probably see a doctor to rule out other causes. Thankfully, breast tenderness is not associated with breast cancer.