I was anti-beer for a very, very long time as well (until virtually six months ago), though coffee has always been my vice.
I'm Irish, and I'm fairly young (approaching twenty soon, which puts me very far in the minority on these matters. ;))
Mostly, I simply don't see the point. I understand the need to socialise and the like, and the need for friends. However, I think that if I make friends when I'm in a different state of mind to my ordinary, day-to-day thinking, then are they really my friends?
Furthermore, I don't like having a crutch. I don't think anybody truly needs to get drunk/high/drink a pot of coffee in the morning to get them through the day. If you do, you've probably made some bad decisions and you should think long and hard about where you're going in life.
I also simply hate the stoner mentality. Why the hell would you want to just coast through life? I mean, make something of yourself! Make your parents proud!
I know, however, that I'm painting a large number of people with the same brush here. I know that many of the people who smoke weed sometimes aren't stoners. I have almost no problems with people who smoke weed every once in a while, just as I don't have problems with people who go to the pub sometimes or with people who drink the odd coffee. I do sincerely have problems with people who smoke pot every other night, or every time they hang out with people, or every time they get even a little stressed. (Medical marijuana doesn't count, though, provided naturally that it's actually necessary. I'm not going to judge people when they're in huge amounts of pain.)
The thing is, though, that past this little speech, I can barely defend my arguments. It's not that I just don't get it (though that's definitely a factor), it's also that I simply have an irrational hatred of smoking.
Go easy on me in your rebuttal, I'm only meek. :)