I believe the Lord is spelling out a simple theological and psychological truth: The self righteous, legalistic Pharisee does not think he sinned and so has nothing to confess; this makes him surly, arrogant, self sufficient and ignorant of his need of Jesus' grace and forgiveness. This is in direct contrast to the sinful woman who was acutely aware of her sinfulness - hence her actions and expressions of love.
Thus, Jesus is saying that the more aware we become of our innate sinfulness, the more we love Jesus and realise our need of Him. (The thirstier the man, the greater he loves the drink of water!)
Most commentaries suggest the same idea.
Ellicott: " … not that the sins of the Pharisee were few, but that he thought them few, and that therefore the scantiness of his love was a witness that he had but an equally scant consciousness of forgiveness."
MacLaren's Exposition: "self-righteous man … He [Simon] is a fair specimen of the whole of them. Respectable in life, rigid in morality, unquestionable in orthodoxy; no sound of suspicion having ever come near his belief in all the traditions of the elders; intelligent and learned, high up among the ranks of Israel! What was it that made this man’s morality a piece of dead nothingness? What was it that made his orthodoxy just so many dry words, from out of which all the life had gone? What was it? This one thing: there was no love in it. As I said, Love is the foundation of all obedience; without it, morality degenerates into mere casuistry. Love is the foundation of all knowledge; without it, religion degenerates into a chattering about Moses, and doctrines, and theories; a thing that will neither kill nor make alive, that never gave life to a single soul or blessing to a single heart, and never put strength into any hand for the conflict and strife of daily life. There is no more contemptible and impotent thing on the face of the earth than morality divorced from love, and religious thoughts divorced from a heart full of the love of God."
Barnes: "To whom little is forgiven - He who feels that little has been forgiven - that his sins were not as great as those of others. A man's love to God will be in proportion to the obligation he "feels" to him for forgiveness."
Gill's Exposition: "Simon, the Pharisee, whose debts, in his own opinion, were few or none, at least ten times less than this woman's; and he had little or no sense of the forgiveness of them, or of any obligation to Christ on that account; and therefore was very sparing of his love and respect, and even of common civilities to him."
We are all, including the modern Pharisees still among us (and in us) are great sinners - But Christ is an even greater loving Saviour. Only this sense of our great need of Him because of our complete hopelessness, can we begin to learn to love as Jesus loved, John 13:34, 35.