What is the source of true joy?

I will confess to not paying proper attention at Lauds yesterday when I would have first prayed the Sunday collect, this year round; and I didn't catch all of its text at Mass: so by Terce my interest was properely roused.

Deus, qui fidélium mentes uníus éfficis voluntátis : da pópulis tuis id amáre quod prǽcipis, id desideráre quod promíttis ; ut inter mundánas varietátes ibi nostra fixa sint corda, ubi vera sunt gáudia. Per Dóminum.

O God, You who convert the minds of believers to Your single will: grant to them so to love what You command and desire what You have promised that their hearts, amidst the inconstancies of the world, may remain set where is true joy. Through Our Lord Jesus Christ...

St Augustine, pray for us!