Because you're getting HIT IN THE NUTS. Is why.
More importantly, why is it so funny when it happens to someone else?
I will now never ever be able to use/hear the phrase "Drop it like it's hot" without smirking:"In deference to our genetic interests, our bodies subconsciously thermoregulate our balls by flexing the cremasteric muscle and drawing the scrotum up closer to the body when it gets too cold and dropping it when it’s hot."
To demonstrate that I have learned something in my 40-some years, I feel like proposing that Mental Floss follow this up with an article entitled "Why Having a Baby Hurts More than Getting Hit in the Nuts."
And never more so than when a football is involved.
My mom, who delivered two children, one without any drugs, said passing a kidney stone hurt worse.
I have passed a kidney stone (twice) and I've been hit in the nuts (at least twice). Kidney stone by a mile. To review the anecdotal evidence:kidney stone > natural childbirthkidney stone > nut shotBehold your pain champion - kidney stone!
Not sure you're working with a valid sample size.
I'm sure I'm not.
If passing a kidney stone is only half as bad as this scene from Deadwood (language NSFW), I'll take a nut shot every day.
Completely coincidentally, Rob Neyer linked to this glorious 1946 promotional video for the Pacific Coast League -- if you skip to 22:43 and watch as they introduce the first of the three OFs warming up for the Hollywood Stars, you'll see the relevance to this topic.