I was asked this question a number of years ago by a Christian girl I'd known on camp. Everyone around her was smoking it and she was just confused about what the problem was. She kind of felt that she as a Christian shouldn't, was tempted, and couldn't come up with loads of good reasons not to. And it got me thinking really: what is having a joint all about? I've met this issue a number of times subsequently in various forms, including having a guy high as a kite in Bible study (not to be recommended). By the way, it's funny that we don't really talk about drugs (apart from alcohol) more when actually everyone around us is doing them. I guess it's cos we don't meet (more illegal) drugs that much in our little subcultures. Perhaps we need to get out more?
Now, it seems that drug-taking can have a whole range of motivations and contexts:
1. It can be a kind of secular spirituality i.e. a seeking after transcendence and meaning.
2. It can be self-medication, alleviation from pain and escape from trauma.
3. It can be about the appeasing of withdrawal symptoms once dependent.
4. It can be about pushing boundaries and experimentation.
But the key thing in all of these motivations (apart from nr. 3) is that experience is the answer. Drugs are about pure experience - reality, truth, morality, hope, relationships etc are all transcended or relativized (temporarily). Experience is everything. Drugs are, in a sense, the logical end of the postmodernism preoccupation with experience. It's interesting that the postmodern philosopher Michel Foucault was a massive experimenter in all kinds of drugs.
Compare two modern dystopias. Orwell's 1984 envisaged a society controlled by a Stalinist state of Big Brother, but Aldous Huxley in Brave New World envisaged a society controlled by drug-induced pleasure where truth and art have been removed. Which one is is more like us?! Drugs are just everywhere and really are one of the icons of our age. Inner cities are obviously blighted by drugs, but the issue is just as big among City professionals. The point is: any culture giving itself over to drug-induced experiences (and I'm including alcohol here) has got a major problem. This is not just because of physical addiction, violence and crime, but because such a society can no longer be truly free. It has abdicated freedom and truth for 'happiness'. I reckon we're moving more and more towards this - experience itself as the means of transcending the limitations and dissatisfactions of our lives.
"What's wrong with a bit of pot?" One answer is: the only real way to gain transcendence without making myself a slave is with Christ. Pot will always enslave me inside my own head. It not forgive me my sin, nor will it give me eternal life.