The Fourth Sunday of Lent, Laetare Sunday, reminds us of the fruit of the Holy Ghost--joy. Joy is a pure gift from God. We can practice charity or experience peace, but the idea of joy and happiness is something God fills us with because it comes from Himself. Joy and goodness go together. As God is all-good, He is the ultimate object of our desires, and we are truly happy when seeking Him.
There is a difference between "feeling happy" and true joy that is supernatural, and that supernatural joy comes from fulfilling God's will in our lives. When we are completely open to God acting in our lives, He takes care of everything for us. For He desires us to be happy more than we want to be happy.
Joy is also lasting. It may seem like a contradiction in terms, but I think when you are sad it is still possible to experience joy. Because even when we are going through tough times, we can still hold onto the hope and happiness of union with God, which more often than not is achieved through suffering well.
I've always liked this quote: "Serve the Lord with laughter" by St. Padre Pio. It is a much fuller life to take each experience great or small with a smile.