Jesus had stated that all the law and the prophets were dependent on loving God first, and then loving your neighbor as yourself. These truths were already in the law but people missed them. 

This commandment was not new in the sense that it had never been given before, but it was brought up again and given new meaning through the example of Jesus. Jesus not only loved His neighbor as Himself, He loved us more than Himself.

John, who wrote his gospel, later wrote about the new commandment of love. No doubt, he received his inspiration from this teaching of Jesus. John wrote, "Brethren, I write no new commandment unto you, but an old commandment which ye had from the beginning. The old commandment is the word which ye have heard from the beginning. Again, a new commandment I write unto you, which thing is true in him and in you: because the darkness is past, and the true light now shineth" (1 John 2:7-8). He then goes on to speak of loving one another.

This commandment, or the revelation that God gave to man through His Word, wasn't new, but was never clearly seen by men because of the darkness that separated them from God. 

In the light of Jesus' life, the darkness was removed and the old commandment of love became new through the example of Jesus in a way that mere words could never express.