Any man will confirm that a kick to the groin hurts a lot. It is not uncommon for them to compare the pain to childbirth, and although we will not know the answer, it is certain that the pain can be compared to a punch to the stomach from a professional boxer.
The testicles, popularly known as balls, produce about 1,500 sperm every second. That's about 90,000 sperm every minute, 5.4 million every hour and 130 million every day. Every man is different and sperm counts vary, but a typical pair of testicles produces more than a million sperm every day. It only takes one sperm to fertilize a woman's egg, but the average ejaculation (which amounts to about half to one teaspoon of semen) contains 40 to 130 million sperm. If sperm are not released by ejaculation, they are simply reabsorbed back into the body.
If you think about it, a woman also has gonads, ovaries, but why are they so well protected inside the body when the male gonads are outside the body and so vulnerable? The temperature of the testicles has to be about 5 degrees lower than the rest of the body for sperm to be properly preserved.
The testicle on the left usually hangs a little lower than the right one and this is no coincidence. This allows the temperature of one testicle to change without that energy being sent to the other testicle as it would if it were touching. In this way, the body can raise or lower the temperature of one testicle without affecting the temperature of the other. Now this is the kind of advanced heating/cooling system that is needed in almost every office building.
We know that the testicles are extremely sensitive to pain, but they are not themselves as sensitive to sexual pleasure as you might think. It's actually the scrotal sac that holds the testicles that's the erogenous zone. The scrotum corresponds to the vulva and labia in women. There is no way to be 100% sure how similar the sensation is, but "we can guess that the sensations are quite similar in that we know that the distribution of nerves is almost identical. Playing with his scrotum alone is unlikely to give him an orgasm.... but it still feels pretty damn good.