Cheerfulness is a sign of a generous and mortified person who forgetting all things, even herself, tries to please her God in all she does for souls. Cheerfulness is often a cloak which hides a life of sacrifice, continual union with God, fervor and generosity. A person who has this gift of cheerfulness very often reaches a great height of perfection. For God loves a cheerful giver (cf II Cor 9:7) and He take close to His heart the religious he loves.

(Blessed Mother Teresa, The Private Writings of the "Saint of Calcutta")