Several reasons for me, listed in no particular order:
1. Damaged left knee, right hip, 3 locations on spine, neck, and right shoulder from 3 motorcycle accidents.
2. Cerebellar Tonsillar Ectopia (damage to base of skull and base of brain stem, also from the MC accidents). (Yes, I still ride.)
3. Diverticulosis (believe it or not, the MJ actually helps with this, cuz it makes all the salads oatmeal, and fibrous crap I gotta eat taste good).
4. PTSD (from the Nam), Depressive disorder, anxiety disorder. sleep disorder, addiction to and withdrawal from Lorazepam and a couple other niceties prescribed by the VA shrink.
5. Cavernous Angioma (a blood vessel in my brain that occasionally leaks blood, kinda like a mini-stroke.)
Probably a couple others that I'm forgetting, but the most important reason of all is, I'm an old pot-head. I toked up a storm in my youth, but I've been off the weed since about 1980 or so. I just recently had yet another falling out with the shrink and in a fit of frustration I bought a bag on the street for the first time since it was $20 an OZ. Just got fed up with trying to do things the 'right' way (read that as 'right wing' way). As was mentioned a couple other times, I noticed an almost immediate 'cure' from doin' the doobie on some of this stuff, particularly the pain, stress, anxiety, and sleep propblems. That makes it a LOT easier to deal with some of the other stuff.
BTW, I live in a Non-MMJ state, so I guess that puts me in the category of being a happy, healthy criminal.