John Lightfoot on John 15:12

[That ye love one another.] "Every sabbath they added that blessing towards that course of priests who, having performed their service the last week, were gone off. Let him who dwells in this house plant among you brotherhood, love, peace, and friendship."

Our Saviour once and again repeats that command, "Love one another": he calls it 'a new commandment,' chapter 13:34: for their traditions had in a great measure put that command of loving one another out of date; and that particularly by very impious vows they would be making. We have a little hint of it, Matthew 15:5, and more in the treatise Nedarim. See also Matthew 5:43, "Thou shalt hate thine enemy": this rule obtained in the Jewish schools. And upon that precept, "Thou shalt love thy neighbour as thyself," let us see the mighty charitable Gloss in Chetubb. "Thou shalt love thy neighbour as thyself," that is, decree him to an easy death: namely, when he is adjudged by the Sanhedrim to die.

When you consider the frequent repetition of this precept, "Love one another," consider also that passage, Matthew 10:34, "I came not to send peace, but a sword": and then having reflected on those horrid seditions and mutual slaughters, wherewith the Jewish nation, raging with itself in most bloody discords and intestine broils, was, even by itself, wasted and overwhelmed, you will more clearly see the necessity and reasonableness of this command of loving one another, as also the great truth of that expression, "By this shall all men know that ye are my disciples, if ye have love one to another."