John Gill

This is my commandment, that ye love one another,.... Christ had been before speaking of his commandments; and he mentions this as the principal one, and to which all the rest may be reduced; for as the precepts of the second table of the moral law may be briefly comprehended in this one duty, love to our neighbour, so all the duties of Christianity, relative to one another, are reducible to this, by love to serve each other. This was the commandment which lay uppermost on Christ's heart, and which he knew, if attended to, the rest could not fail of being observed. The argument by which, and the manner in which, he presses it, is as before: as I have loved you; than which nothing can be more strong and forcible; see Joh 13:34.