Most people take drugs to feel good, and certain drugs can derive feelings of euphoria and pleasure. There are separate and individual effects from every drug that is being used. For example, people who smoke pot may obtain a sense of liberation, and they can find everything to either be funny or mentally stimulating. For others, they might feel paranoid and overeat.
The fact remains that the person made a conscious decision to alter their current state of being. Certain drugs may benefit those who are being treated for depression, anxiety, ADHD, or social phobia. It depends on if a doctor is monitoring the person or if the person goes off on their own and treats themselves. Curiosity may also play a role in someone who chooses to take drugs, and this is why teenagers are so susceptible to getting drunk or high at parties.