It is replicating in our own lives what God has already done for us. It is sin that separates us from God. Justice would be to destroy sin by destroying the sinner. That would be justice. Fortunately God does not give us the justice we deserve, but out of His Love for us, He gives us Grace.
Grace is pure gift.
God sent his only begotten Son to the end that all who believe in Him would not perish, but be salvaged. That is what God has done for us. He asks that we do it to one another. That, my friends, is why we are not only commanded to love one another, to love what is lovable, but to also love what we find to be unlovable and to love our enemies. To the degree we can do that will be the degree we will find that we also will intensify and deepen our love for those we love most and for God who himself is love.