In verse 8, Paul has presented us with an interesting paradox. On the one hand, he states that we should owe no man anything that he can rightfully claim from us, yet on the other hand, we must owe everyone more than we can hope to pay—perfect love. By this, he extends and intensifies the concept of obligation. We must be more scrupulous within the limits of the customary concept of indebtedness, and we must infinitely widen the range within which they operate.
Was it not our failure to meet our obligations to God and man that accrued the unpayable debt in the first place? Now that the debt has been paid, we are under obligation, not only to strive to avoid falling into the same trap, but to expand and perfect the giving of love. The paradox is more apparent than real because love is not merely one's duty added to others, but is the inclusive framework within which all duties should be performed. Love is the motivating power that frees and enables us to serve and sacrifice with largeness of heart and generosity of spirit.
However, as long as we view love merely as the keeping of God's laws, we are stuck on a low-level, letter-of-the-law approach to righteousness. That is most assuredly a vital and necessary aspect of love, but there is far more to love. That level of love can be merely one of compulsion, and be done in a "just because" attitude: "I must love this person, but I don't have to like them." This may suffice for a while, but Paul, by drawing upon Christ's teaching, unveils an entirely new significance to the concept of obligation.
Of what level was the love of the fallen woman who washed Christ's feet with her tears, wiped them with her hair, kissed them with her lips, and anointed them with costly oil? Was her conduct merely to keep a commandment, or was it an exquisite expression of a heart freed to give its all?