In my experience, that's because you're critiquing others and yourself too much. It's not fun if all you see is the shitty stuff or what you need to "improve." Who cares if you make good or bad art? There are plenty of perfect drawings out there that no one has ever seen. It really doesn't matter. In a related note, if you went to art school, it might be the art school hangover from juries. I have a vendor that still worries about what his college teachers/peers might say, and I think he's in his 50s or 60s.