I agree brain chemistry probably plays a significant role. I've avoided stimulants (even as mild as caffeine) and depressants (including alcohol) since I was a teenager. I still have what I call an addiction prone personality, and can happily spend 15 hours/day playing the same game for days or weeks on end.

I am able to direct this "addiction" to more productive tasks as long as I can convince myself the reward is there. There was a period of about a year where I averaged 100 hr work weeks and got very little sleep. I belive various forms of meditation kept me sane during that period.