St. Gianna, Her Life of Joy and Heroic Sacrifice by Giuliana Pelucchi. In it, Pelucchi shares a poem St. Gianna wrote in praise of smiling. Her husband, Pietro, referred to as a "hymn to joy":
Smile at God, from whom all good things come.
Smile at God the Father with ever more perfect prayer.
Smile at the Holy Spirit.
Smile at Jesus when approaching him at Holy Mass, in Communion, in a visit to the Blessed Sacrament.
Smile at the one who represents Christ, the Pope.
Smile at the one who makes God personal, the confessor, even when he challenges you to reject sin.
Smile at the Blessed Virgin, to whose example you must conform your life, so that, seeing you, please might be led to holy thoughts.
Smile at your Guardian Angel, because this angel has been given to you by God to lead you into Paradise.
Smile at your parents, brothers, and sisters, because you must be a torch burning with joy, even when they challenge your pride.
Smile always in forgiving offenses.
Smile [at all the people you associate with], banishing all criticism and murmuring.
Smile at everyone the Lord sends you during the day.
St. Gianna also had this to say on the subject of happiness:
I was always told that the secret of happiness is to live moment by moment, and to thank the Lord for everything he sends us, day by day.