Some major causes of drug abuse include mental illness, poor coping skills, and genetic predisposition to drug addiction. Sometimes, the symptoms of mental illnesses like depression, PTSD, or anxiety become so overwhelming that a person feels they need to turn to drugs to find relief.
For others, it's difficult to cope with the stress of day-to-day life without the use of drugs. However, it's important to remember that drugs can only provide a temporary escape from stress and mental illness, and drug abuse will likely continue until that person learns better ways to deal with their issues.
Sometimes, people find themselves addicted to and abusing drugs because the blueprints for addiction are written into their genes. There are still mysteries to solve about how and why our brains and bodies get addicted to certain drugs.