13 Finally, there is one motive which We are unwilling to pass over in silence, personal to Ourselves it is true, but still good and weighty, which moves Us to undertake this celebration. God, the author of every good, not long ago preserved Our life by curing Us of a dangerous disease. We now wish, by this increase of the honor paid to the Sacred Heart, that the memory of this great mercy should be brought prominently forward, and Our gratitude be publicly acknowledged.
Source: Annum Sacrum (Vatican.va)