"Adam fell that men might be, and men are, that they might have joy".
Joy is more than happiness, it encompasses your entire being. Joy is an eternal emotion. Unlike most emotions, joy has the ability to last fully for long periods of time. The purest and most bright feeling I have ever experienced is sure joy in the gospel and in my Savior. I know that this gospel is the light and the only way to experience everlasting and eternal joy. I am so grateful that I have this knowledge. I recognize the immense blessing that it is for me to be able to feel joy now, and if I choose wisely, forever.
"In order to have joy, you need to understand that, as a child of your Heavenly Father, you inherited divine traits and spiritual needs--and just like a fish needs water, you need the gospel and the companionship of the Holy Ghost to be truly, deeply happy. Because you are the offspring of God (see Acts 17:28), it is incompatible with your eternal nature to do wrong and feel right. It cannot be done. It is part of your spiritual DNA, as it were, that peace, joy, and happiness will be yours only to the degree you live the gospel."
-Marcus B. Nash Ensign, Nov. 2006, 49