A few days ago, my husband was doing our nightly scripture reading with the kids. He read Matthew 5:16 "Let your light so shine before men that they may see your good works and glorify your father which is in heaven." He asked the children if they knew what the light was that they were referring to. To illustrate, he hid his face in his hands peek-a-boo-style, and then opened his hands to reveal his bright shining smile. This registered with the kids right away. The light that we are to shine for others to see is the light that is inside each of us. It is the light of Christ...the light that is given to everyone who enters the world.
As we cultivate the light of Christ that is within each of us, our spirits are more capable of being joyful.
But how many people do you know who are truly joyful? This life can really weigh on us sometimes, making it difficult to experience joy, even when we are doing what we should. In the 1987 article Do Not Despair, The First Presidency explains possible reasons for this, and outlines 12 keys that can help us overcome depression and discouragement. I found a lot of the suggestions helpful, perhaps you will too.