I think for me it started when I noticed a lot of my friends wore boxer shorts. Shorts back then were short (!) and the boxers would often be a bit longer and show at the leg openings of the shorts. This alone turned me on. Add to that the shorts were also worn tighter, so there was always a bulge. And not just a bulge, but a bulge that sometimes outlined the penis and balls separately. If the guy was sitting down, there was almost always a chance, if the just-right angle presented itself, of catching a glimpse up the inside of his leg, and seeing a bare ball or penis head. Which would make me chubb up a bit, enhancing my own bulge.
So I started wearing boxers, and I just loved the feeling of my bits being loose, and that just led to going commando quite often. I still love that loose feeling. I also love the exhibitionist side, especially when I'm wearing shorts.