I had the fortune of attending a private acoustic performance and meet-and-greet with STP on New Year's Eve (2002?). During the meet-and-greet, I remember Scott was very reserved. In fact, there was a huge contrast between how engaged and friendly the other three were and how reserved Scott was. At first, I wondered if it was an ego thing, but then as I declared them the greatest band in the world, Scott's face lit up with a smile and he gave me a sincere thank you.
Having suffered from social anxiety myself and self-medicating with drugs and alcohol for many years, I honestly believe this is a big part of the picture. Social anxiety is an embarrassing thing! Hell, some people still don't even believe it's real. I had it for years as a teenager without understanding what it was; I just knew something was wrong with me. Then it took several more chaotic, drugged-out years before I was able to start learning to manage it (without drugs).