It's kind of funny when people ask "Why does this happen?" I'm guessing it is directly related to you smoking something. Marijuana affects people differently. For instance, whenever I'm really zoned out I grind my teeth. I know I do it when I smoke, I can't stop it... but I know it is from smoking. I also know that if I quit smoking, I will probably grind my teeth less. I don't feel this is annoying enough to make me quit.
Man that is so wierd because I could swear my teeth have been slowly shortened over time from this phenomenon called teeth-grinding. Maybe we're just some hungry mofo's but you know that ain't right.
Second thing I wanted to say, as this happens to be my first post on this site (signed up the other week), I figure this is as good of a time as any to say a big warm HELLO and big up all you smokers out there, let's keep it real and keep burning that good shit.