Outward happiness is a testament to the strength of a Mormon’s faith. Heavenly Father has given a plan of happiness (or plan of salvation) so a person “can be happy in this life and receive a fulness of joy in the eternities.” (Gospel Topics: Happiness, lds.org)
“As you seek to be happy, remember that the only way to real happiness is to live the gospel. You will find peaceful, eternal happiness as you strive to keep the commandments, pray for strength, repent of your sins, participate in wholesome activities, and give meaningful service.” (True to the Faith, p. 80)
When Mormons are faithful in carrying out the work of the LDS Church and following the commands that lead to eternal progression, they will find lasting happiness both in this mortal life and throughout the eternities. A happy Mormon gives an outward testimony to how well they are successfully obeying the LDS gospel.