PMS - PMS or pre menstrual syndrome is probably one of the most common causes of tender breasts. During this time of the month a woman's body goes through unbelievable changes. All of these changes cause a huge hormonal imbalance known as estrogen dominance. So you basically have too much estrogen in your body at the time. Because of that you can retain water and when the water sits in the tissue of your breasts, it can cause pain and tenderness. Sometimes you can't even lay on them. Trust me I know how that feels and it is not a good feeling at all.
Premenopause and Menopause - As you get older things start to change in your body. You no longer have a regular cycle. Your body will gradually start to slow down the production of estrogen and progesterone. Before menopause 15% of your cycles with be anovulatory which means that no egg is released. Most of the progesterone in your body is produced after ovulation. Since you will no longer be ovulating, you will have almost no progesterone at all. Because of this, your hormones will be out of balance. And you will probably still be estrogen dominant which will lead to sore and tender breasts.
Being Overweight - Most people don't realize it, but being overweight can have a huge affect on your breasts. Not only will they get bigger and probably start to sag, but they will also start to be tender and sore more ofter. Again this is due to a hormonal imbalance. When you are overweight, the levels in your body are all over the place. And because of this your body will retain water. And your breasts are a very common place for water to sit. When the water get in the tissue it because heavy and it weighs down on the breast. This in turn will make them very sore and tender. So sore that you might not even be able to touch them.