There are three theories that explain drug use – the biological, psychological and sociological theories. In terms of the biological theory, some people might inherit certain things from their parents, such as extreme allergies, which would make them turn to drug use to combat the symptoms of their allergies. In terms of the psychological theory, some people might truly make themselves believe that they have all kinds of symptoms of all types of different things that could be wrong with them, for which they turn to drugs to remedy their so-thought illness. This can be especially common among hypochondriacs.
Regarding the sociological theory, people often use drugs such as tobacco and alcohol because all of their friends or families use them as well. Being constantly exposed to these drugs by people you trust and care for makes you think that you should follow along and use them also. Society plays a huge factor in how people choose to make certain decisions in life, such as whether or not they should drink or smoke.
If illicit drugs were decriminalized, the result would have a large impact on drug-related social problems. There would likely be more widespread use of illicit drugs because people who were previously afraid of the legal risks of using these drugs would no longer fear the potential consequences if they were caught using them. Another example of the potential impact would be that prison populations might slightly decrease because there would be fewer drug addicts and/or small-time dealers being locked up for drug possession.