I still to this point do not understand why people need to use drugs to have fun. And yes, I am including alcohol in this. Both, I understand, are not chemically addictive, however, when people are high/drunk, they don't feel the need to be responsible for anything. Literally everyone I've seen get high/drunk doesn't care about important stuff while they're doing so. They're also not learning anything while in such a state, which indirectly makes them dumber. I live with people who do this stuff. I know plenty of people who do this stuff. I can see the difference between people who do and don't. There is a correlation, and it's not low.
Though, it could be backwards. Perhaps the drugs aren't making the person stupid. It's the stupid making the person use drugs.
While you may have had a different experience, not all people need drugs to have fun. It's just a fun thing to do. Experimenting with a substance that alters your perception in a minor way shouldn't be criminalized. Nor does everything you do have to be learning. I don't have any experience with drugs/alcohol, but from what I understand, being drunk does not change your reasoning ability that much, and anything stupid you do during drunkenness is simply something that you would want to do when sober. But I can't really back that up.