It was considered a new commandment  given to them because they were used to the laws of Moses  which believed in an eye for an eye. In some cases you were required to kill your enemyf he tried to kill you and not love you enemy or your bad neighbour. Jesus loved us unconditionally but it was not easy as human beings to love one  another unconditionally. He even eat and talked with sinners, this was quite difficult to accept by the Pharasees. The most important thing is that if this is done, all men will know that we are his disciples.

The most important thing is that Love was his greatest commandment. A new commandment. And it was that same reason that made God give Christ his loved son to die for our sins.