your chief study

Let your chief study be to acquaint yourself with God, because it is the only knowledge which can fill the heart with a peace and joy which nothing can disturb.

~St. Elizabeth Ann Seton

if you're happy and you know it

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....

~ Mother Teresa (from the book, Come, Be My Light)

joy - gotta have it...

...St. Thomas Aquinas says expressly that 'everyone who wants to make progress in the spiritual life needs to have joy.'

Sadness debilitates us...

This interior joy is also the state of mind necessary for perfectly complying with our obligations.  And the greater these are the greater must be our joy.

The greater our responsibility (parents, teachers, priests, superiors...) the greater our obligation to have this peace and joy to give to others, and the greater the urgency of recovering it when its habitual possession has been interrupted or disturbed.

In Conversation With God: Vol 2 p. 304