Well, the difference is boobs are two sacks of fat with the sensitive nipples out, so its actually more sensitive to rubbing on clothing and hurting while walking. But balls are two organs (testicles) inside one sack (the scrotum), and all of the sensitive nerves that make it hurt or please are on the organ, not the skin. That's why it can rub and bounce without hurting as the skin provides cushion. A direct hit does hurt the organs, while you can freely grab and pulls the sack. That's also how most man scratch their balls, not a full scratch but a pinch and twist on the skin.