Drugs feel amazing. Getting high is like the fucking grand canyon, one of the things in life that lives up to the hype. Doing drugs makes you happier than you thought you could be, and there are a lot of people who don't have a lot of reasons to be happy.
We shouldn't pretend that drugs are bad, mm-kay. Drugs are awesome. That's the problem. They're too awesome. It's an awesome overload, and you end up not wanting to do anything except for drugs.
People who do drugs are not evil. They're having fun, experiencing new things, making friends and bonding over shared experiences. You tell a bunch of kids that drugs will ruin their lives, and then somebody at a party passes them a joint or offers them a bump of coke, they're going to realize you were full of shit.
Like, let's say that there was some weird flesh-eating bacteria that was specifically found only on water slides, but only on a few water slides. Now it's your job to convince all the children of the world to avoid water slides, because of the small possibility of bacteria. It's a serious problem, and it would be correct to tell everyone to avoid all waterslides everywhere, even if only a small percentage of waterslide riders died horrible deaths. So you tell people waterslides might kill you or maim you in excruciating ways. But if you act like waterslides aren't fun, you lose all credibility. Most people who ride the waterslides don't die, and they go on to tell everyone how much fun they had on waterslides, and that doesn't make them bad people.