Staying stoned 24/7 will do that to you. Not even counting the other crap he regularly does.
Don't get me wrong. Weed is fun, and mostly harmless, but I've never ever understood those people who treat weed as a lifestyle choice. You know the people I'm talking about. The folks that can rattle off complex chemistry (as long as we're talking about hydroponics), or fluid dynamic mathematics (only as it relates to bong construction), but will give you a blank stare & a "Duuuude...what?" If you ask them where their keys are.