“The commandment is not new because it differs in content from what can be found in the Jewish scriptures (cf. Lev. 19.18), but because it forms part of the new revelation and the new order that has come with Jesus. It is therefore grounded in his own love for his followers, which entails his death, and it is shaped by and participates in the relationship of love between Jesus and the Father. In particular, Jesus’ love for his own (cf. 13.1) serves as the foundation and model for his disciples’ love for one another. After Jesus departure, the carrying out of this command will be the sign of the continuation of his cause and his mission in the word: By this all will know that you are my disciples, if you have love for one another. The identifying mark of the community Jesus leaves behind is to be the mutual love of its members.
This quote is taken from Andrew Lincoln’s excellent commentary on John’s Gospel (Peabody: Hendrickson, 2006), 388. I used this commentary when I was working through John; I highly recommend it along with his other work on John’s Gospel called Truth on Trial.