In my case, a have a terrible zipper phobia. When I was in 3rd grade, my dad sent me to school without underwear (Mom was sick, and didn't want to do the laundry.) Well, I went to the restroom at school, and got "it" caught in my zipper.
I went back to my classroom, and had a very nervous discussion with my teacher (Mrs. Burton) and had to go with another boy to help me unzip. Which means it had to be yanked down like ripping off a band aid. That hurt like hell!!!
So, I vowed never again to go anywhere without wearing underwear. Guys, who don't wear underwear just tell me to buy button fly pants. Tried that, and didn't like it. So, I've always worn underwear. No more zipper accidents since.