Gregory the Great — as excerpted in the Catena Aurea on Luke 15:1-7

“(in Hom. 34.) He placed the sheep upon his shoulders, for taking man’s nature upon Him he bore our sins. But having found the sheep, he returns home; for our Shepherd having restored man, returns to his heavenly kingdom. And hence it follows, And coming he collects together his friends and neighbours, saying to them, Rejoice with me, for I have found my sheep which was lost. (1 Pet. 2:24, Isai. 53.) By His friends and neighbours He means the companies of Angels, who are His friends because they are keeping His will in their own stedfastness; they are also His neighbours, because by their own constant waiting upon Him they enjoy the brightness of His sight.”