My guess is that, at least when it comes to Marijuana, conservatives are so deeply offended by drugs because they're anti-productivity. Pot slows you down, relaxes you, lowers drive (even just temporarily). For those on the right wing, this translates to, "it makes you lazy, a burden on society, it undermines the American spirit of hard work and self-started progress". They rationalize it any number of ways, but I think that aspect is what makes it personal.