Joy is to be thought of as the true source of that liberty of mind which alone is able to unite the apparently incompatible qualities of the spiritual life, giving a freer rein to natural expressions of love while remaining inseparably attached to mortification. In our joy we must be careful to keep our spirit mortified, and practise mortification in order to increase our joy.
I must therefore remain always and invariably happy, while never for one moment desisting from self-denial. It is self-love which stunts the growth of the spirit and saddens us; self-denial restores life, serenity and peace.
--Pope St. John XXIII, Journal of a Soul