I think it stems from my hardcore perfectionism.  I could literally take the same constructive criticism in a volunteer setting and paid setting and react completely differently.  At work I have always been so petrified of messing something up that I get paranoid, start having dozens of panic attacks a day, then stop showing up--usually without even making tiny mistakes.  But doing that same thing for free?  Sign me up!

It's amazing to me...  Absolutely incredible...  That people can hold down jobs and work for the same places for YEARS.  I think the closest I ever came to even a year at the same place was a little shy of eight months.