There's one type of testicular pain that will result in these words coming out of a doctor's mouth: "The good news is there's nothing wrong with your testicles. The bad news is that you have a kidney stone."
This illustrates another important reason why you should never ignore testicular pain: It may lead your doctor to discover a related health issue.
Though the source of the pain -- the kidney stones -- doesn't originate in the testicles, it can feel like it because the pain projects downward. These stones are compacted masses that develop in your kidneys from urinary crystals. Doctors aren't sure exactly why they form, or why some men are more susceptible to them than others.
Most of the time, you can pass the stone without treatment (though pain medication is often prescribed). You'll drink lots of water -- and then deal with the pain that comes when the small stone passes from your kidney into your bladder and then exits through your urethra.