There is joy in being able or allowed to go all out to do something. For example, you get the chance to do all the enjoyable things over a weekend or play a sport with all your heart and strength. Or it could be that you have the opportunity to express yourself fully in some way or even eat your favorite foods to your heart's content. In the same way, if you can let go entirely in God, you will feel great joy in your soul. This joy comes from feeling free and knowing that you have pleased God and made spiritual progress yourself.