I do not know why you are worrying so much about this. Do you really have pain or discomfort about your testicles, or just you feel they are sensitive when you masturbate? We all do know from the time we were young that the testicles are very sensitive. Is not that known how good swift kick in the testicles causes a person to double over in pain? To tell you, for the starts, that like the penis, the testicles come in all shapes and sizes. They are wrinkly and may be smooth or hairy, depending from person to person. Even though there are two testicles, we all should treat them as a unit. They go hand in hand and do not at all liken to being separated and yanked apart, which you might tried and feel pain. Testicles are needed for gravity, and they will probably descend a little, as you getting older. Conversely, they stick close to the shaft when a man is about to climax. This might happen and when the outdoor temperature gets very cold. Incidentally, the smooth spot on the underside is a great place to touch and stimulate, because they are so sensitive. As you can see, that is normal and no reason to worry.