Donald Carson reflecting on Jesus’ command to love one another points out the standard of that love is his own sacrificial death on the cross. This helps explain why Christians as they mature find that they are able to see their sin more clearly. As the cross comes more into focus, so does our need of that cross.
“The more we recognize the depth of our own sin, the more we recognize the love of the Saviour; the more we appreciate the love of the Saviour, the higher his standard appears; the higher his standard appears, the more we recognize in our selfishness, our innate self-centeredness, the depth of our own sin. With a standard like this, no thoughtful believer can ever say, this side of the parousia, ‘I am perfectly keeping the basic stipulation of the new covenant.'” (D. A. Carson, The Gospel according to John, page 484.)
This also explains why it is so astounding to hear anyone claim that they are without sin.