I have a very simple world view which, I guess, could be considered a radically liberal one: everyone can do as they please as long as it doesn't tread on the same rights of another person. You want to inject, snort or sniff something? Go ahead! It's your body, after all. Neither I, nor any government should have anything to say about this. Obviously, you should not operate a vehicle while intoxicated and we need to…
But I think popularity is probably the biggest factor. Its when you see your own family/friends' family being destroyed by criminal convictions resulting from marijuana use does it create a strong push to legalize the stuff.