"I am so disappointed when I meet nuns who are joyless, who may smile with the smile of a flight attendant but not with the smile of joy that comes from within," the pontiff told nuns at the Santa Chiara cloistered convent in the Italian city of Assisi.
Living in an enclosed order must not be "purgatory", Francis told the nuns.
"Nuns must not be too spiritual, and must endeavour to be experts in humanity in order that convent life is not purgatory," he said.
He urged the nuns to embrace communal living and to live together like members of a happy, loving family.
"Know how to forgive, how to tolerate each other because the devil takes every opportunity to divide us. Nurture friendships with each other and family life. And don't brag!" he told the nuns.
Earlier, 50,000 people packed into the square outside Assisi's medieval Basilica of San Francesco to hear Francis celebrate mass.
The charismatic Argentinian pontiff was on a one-day pilgrimage to the Umbrian hillside town to pray at the shrine of the 13th-century saint whose name he adopted when elected earlier this year.
Francis also met groups of poor, sick and disabled people who were being looked after by Catholic orders or charities.
The Pope said he wants to see an overhaul of the Church, bringing it closer to ordinary people.