There are a multitude of reasons behind why people continue to engage in these activities despite the dangers of doing so. Addiction is very common, and all the things you mentioned above come with the risk of addiction. Unfortunately, many people fall prey to the influences of others, especially in their youth. It is very, very difficult to break an addiction. Many people may understand how hazardous it is to smoke and do drugs, but knowing it is bad is only half the battle. Even people who seek rehabilitation and are willing to quit these habits struggle to overcome their addictions. The chemicals in these substances alter brain chemistry and in a sense, trick your mind into believing your life depends on it. You cannot always fault people who do these things. Many of them are innocent. Prescription drug abuse tops this list. It would be unethical to deny people suffering from pain the relief they need to feel comfortable. Even some medications used to treat mental illness have a risk for abuse, and people who have mental illness, such as depression, are unfortunately more likely to abuse these drugs the most. It’s a very sad thing to witness. These people seek help for their physical or mental pain, only to run into more pain and suffering.