One a person tries and gets hooked on alcohol and drugs and they like the feeling they get, they want more and more of a bang for their money so they abuse it . Many not realizing what is happening and then its too late, and many go beyond abuse which leads to death by the abuse
Unless someone has you tied down and is pouring alcohol down your throat or injecting drugs into your arm days on end, it's all up to the person to CHOOSE to never try it.
Past that, there are other factors such as genetics which cause some to get addicted more easily, involving the genome..which is the information surrounding the gene which is released when the substances are tried.
It's a lot more complicated than that, but many studies can be found on internet under "genomes and addictions".
Many things can contribute to someone abusing drugs or alcohol. Environmental factors, such as they are raised in such a place that is accepted.
Mental and emotional illness and the people self medicating.
Genetic factors that make some people more susceptible.
Most of those who I have known to abuse had something horrible happen in their lives and they turned to alcohol or drugs to dull the pain. Some quit when they realized that it wasn't making anything better and made most things worse.
Government crowd control.