The Vilna Gaon explains further that simchah is the inner state of happiness, rather than its outward expression. The Maharal explains that simchah describes the emotion one feels when one is complete and does not lack anything, while sasson is the momentary joy one experiences for gaining something or achieving some sort of renewal. Before a child's life is clouded with doubts, before he is frustrated by unfulfilled drives and ambitions, he is always happy, for he does not lack anything. But this state does not offer us an ideal to strive for. To be without drives or ambitions is to remain a child all one's life.
Life is full of needs and challenges, which, by their very nature, create vacuums to be filled.
The source of the happiness described as simchah lies in enhancing one's awareness of God and His providence, for with this awareness, one feels more complete. A person is beset with shortcomings and frustrations only because he considers himself a separate entity, unattached to God. Then his shortcomings are indeed shortcomings, and feeling that he is missing something is a true indication that he is genuinely lacking in an essential aspect of his life. Not so the person whose life is infused with faith and a keen awareness of God. There is no joy, our Sages say, like the resolution of doubt.