ANDY'S ANSWER: Probably close to half of boys notice some breast enlargement during puberty. This is called gynecomastia and is perfectly normal. Usually, by the time you are are an adult, your breast or breast will have gone back to normal.
What actually causes your breast tissue to grow is hormones. Both sexes produce estrogen which is what causes breasts to start to grow. Boys just produce very small amounts of it. Sometimes though, they produce enough to stimulate breast tissue to grow. This can happen to one or both breasts, and they also could feel a little tender, just like what happens to the girls.
Besides puberty, sometime using certain medications or drugs like marijuana or steroids can cause a hormone imbalance and trigger gynecomastia. If it's puberty, in most cases it will go away on its own, but if not, you should check with your doctor.