Gregory the Great
Catena Aurea · Johannesevangelium 15
This is my commandment, That ye love one another, as I have loved you. Greater love hath no man than this, that a man lay down his life for his friends. Ye are my friends, if ye do whatsoever I command you. Henceforth I call you not servants; for the servant knoweth not what his Lord doeth: but I have called you friends; for all things that I have heard of my Father I have made known unto you. Ye have not chosen me, but I have chosen you, and ordained you, that ye should go and bring forth fruit, and that your fruit should remain: that whatsoever ye shall ask of the Father in my name, he may give it you.
(Hom. xxvii. in Evang.) But when all our Lord’s sacred discourses are full of His commandments, why does He give this special commandment respecting love, if it is not that every commandment teaches love, and all precepts are one? Love and love only is the fulfilment of every thing that is enjoined. As all the boughs of a tree proceed from one root, so all the virtues are produced from one love: nor hath the branch, i. e. the good work, any life, except it abide in the root of love.