There are two reasons why Jesus calls "Love One Another" a "New" commandment: (i) The statement summarises the whole Mosaic ten commandments, as who ever loves God will show same through his love to fellow men, which will manifest in his dealing with his neighbours. This exactly is what Mosaic commandments instructs. (2)Moses had earlier approved that Israelites could revenge if anyone offended them; but Jesus' new commandment disapproves this, and approves total forgiveness, irrespective of the gravity of such offence; and this can only manifest in our 'love for one-another'.