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.
Once you make a decision, the universe conspires to make it happen.
Ralph Waldo Emerson
You can resolve to live your life with integrity. Let your credo be this: Let the lie come into the world, let it even triumph. But not through me.