John 13:34 A new commandment I give to you, that you love one another; as I have loved you, that you also love one another.
Mat 22:37-40 Jesus said to him, “‘You shall love the Lord your God with all your heart, with all your soul, and with all your mind.’ This is the first and great commandment. And the second is like it: ‘You shall love your neighbor as yourself.’ On these two commandments hang all the Law and the Prophets.”
Our Lord told us to love God with all our heart, soul and mind, and to love our neighbour as ourselves.
Lev. 19:18 You shall not take vengeance, nor bear any grudge against the children of your people, but you shall love your neighbor as yourself: I am the Lord.
Loving our neighbours as ourselves is the old commandment.
1 John 2:7 Brethren, I write no new commandment to you, but an old commandment which you have had from the beginning. The old commandment is the word which you heard from the beginning.
What is this new commandment?
1 John 2:8 Again, a new commandment I write to you, which thing is true in Him and in you, because the darkness is passing away, and the true light is already shining.
The wicked Jews (in darkness) abused the old commandment by giving excuses. (The priest and levite in the parable of the good Samaritan; or if I am starving, I can survive on rice alone, so I will just give this starving family only 1 kg. of rice, for this is how much I love myself). So when the true Light comes, this darkness will disappear. When our Lord decided to suffer and die for His disciples (mankind), He repeated this new commandment to us again.
John 15:12-13 "This is My commandment, that you love one another as I have loved you. Greater love has no one than this, than to lay down one’s life for his friends."
Brethren, we have to love one another as Jesus out of His love for us, died for us (please see the love of Jesus in 'Passion of Christ'), so that we too should be willing to die for one another.