The easiest answer for me is that I am an insanely competitive person. It's not that I have to win (though I prefer it), but I do actually have to get better. I love getting better - and showing is such a fabulous (and tangible) way to mark improvement, frame goals and feel a bit of validation
Beyond even that I love the community. Sure there are some bratty people who aren't exactly welcoming, but even with the stigma of "Show Barns" I've found that the vast majority of folks, at least in our area, really just want to geek out about horses with anyone willing to geek out with them. People like to chat at the back gate, competitors comment on improvements from the last time they saw you out, people share their morning pastries and afternoon beers and just want to enjoy the event.
That's part of the fun.
Also, I do love ribbons and score sheets. I miss getting dressage tests back and scouring the sheet for every bit of praise and criticism and heading home with a clear sense of what to work on, but really, ribbons sort of do the same thing. It's easy on the drive home to admire a big Tri-Color and know that we (mostly) earned it, or to stare at the 6th place Eq ribbon and know what I need to work on to change it (seriously? you sit a lengthened trot??).
So I love showing. It exhausts me and sometimes The Boy questions the it a bit when I'm furiously stewing on a bad round at the back gate, but by the time we haul home, unload and take the braids out, I'm always fulfilled with the effort and experience even if the pocket book is screaming a bit in pain.