Jesus Christ did not come to teach us that we should love one another. We already knew that. What He came to do was teach us that our conception of love was entirely inadequate.
To achieve this, He came first and foremost to show us how much He loves us; for unless we first understand that, we can never live up to His conception of love.
In other words, we cannot adequately love from a standing start; we must first accept His love, and then reciprocate it.
If we ever lose that sense of His love for us, we should seek to regain it before attempting to love others as He commands us to love. Only in the assurance of His love for us can we find the strength to turn the other cheek to all those, near and far, who are slapping it.