I think it's all about pleasure and pain, they are calling it happiness, but I think people use substances to feel good and that makes them "happy". They start wanting to stop or start questioning their motivations when the consequences start piling up and it begins to bring pain and discomfort to their lives. When the pain of continuing becomes greater than the pleasure they get from getting high, they quit because they believe it will take away the pain that using is now causing in their lives.