The use of psychoactive drugs is not something that appears out of nowhere.
People have many reasons for using drugs.
Keep in mind that drugs are chemicals and complex molecules, just like the chemicals and neurotransmitters that shape your everyday behavior. Drugs are categorized based on the effect that they have on the nervous system. Depressant drugs may be inhibitory and reduce the activity in a neuron. In the case of alcohol, cells in the brain that normally inhibit or control behavior are themselves inhibited by the alcohol. At low doses this appears as increased energy and attention.