Self motivation is like a muscle, it needs exercise to keep strong and in good shape. If you fail to exercise a muscle it will gradually atrophy until it becomes completely useless.
If you allow this to happen, it takes a great deal of extra effort to restore the muscle to full power and to regain your previous fitness level. Self motivation is needed when the wear and tear of daily life slows you down and you need to give yourself an extra push to keep going.
Getting into the habit of daily exercise will ensure your motivational muscles won't let you down when you need them.