The smile that covered a "multitude of pains" was no hypocritical mask. She was trying to hide her sufferings - even from God! - so as not to make others, especially the poor, suffer because of them. When she promised to do "a little extra praying & smiling" for one of her friends, she was alluding to an acutely painful and costly sacrifice: to pray when prayer was so difficult and to smile when her interior pain was agonizing.
Brian Kolodiejchuk, Mother Teresa: Come Be My Light: The Private Writings of the "Saint of Calcutta"
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