People instinctively self-medicate all the time. Certain fungal infections can be treated by drinking milk, and strangely enough people with this infection will begin to consume a lot of milk without even knowing why.
It takes more than a drug to become an addict. People can even do cocaine a few times and walk away from it. Marijuana is not addicting at all, but some are habitual.
People use what drugs their body feels they need. Some people want to be slowed down, others want to be sped up. You never watch an episode of Intervention and see a person with anything but a miserable story. Not once has someone had a perfect existence. They all have something destroyed as a kid they can never get over. Feelings of massive insecurity or anxiety from what has happened. Something they can self-medicate.
Drugs don't make addicts.