It is rare for people to start smoking after the age of 18 – but it happens. One of the main reasons people start smoking is stress. Adults experiencing some certain levels of stress -- in a high-pressure job, for example -- may start to smoke as a way to manage the tension and nerves associated with the situation. Another reason is to self-medicate. For some, smoking is an essential way to self-medicate for illnesses that cause pain. Other people believe it can help relieve some symptoms of mental illnesses.