Why should people stop take drugs?

Depends on the drug, and how it is used. Drugs, like alcohol, when used in moderation are harmless, even beneficial. But if you find you cannot control your drug use, when you become an abuser, then perhaps you should stop. This applies to any activity, of course, not just drugs.

But even taking drugs that are known to be physically harmless, whether addictive or not, can cause problems for an individual in a political environment of prohibition. Black market prices, criminal prosecution, and unknowable quality of product, are just a few of those.