The woman perceived a greatness in Jesus that motivated her to so abase herself. A proper sense of obligation works to produce a valuable Christian virtue—humility.
Notice her emotion, devotion, and seeming unconcern for public opinion in going far beyond the normal task of a slave. We can safely guess that Jesus had played a huge part in turning this woman from her bondage to sin. She may have first simply been among the crowds who were convicted by His messages. However, she thought deeply and personally on the difference between her life and His words. When she heard He was nearby, she rushed to Simon's home, ignoring the scorn of others to express her gratitude to the One who had set her aright.
Her deed expresses her love and gratitude springing from recognition of His greatness as compared to her unworthiness. She felt obligated to respond in a way so memorable that God recorded it for all humanity for all time to witness. Note that the Bible shows human lips touching Jesus only twice: Here and Judas' kiss of betrayal.
Now notice the contrast with Simon the Pharisee, who was evidently a man of some substance and a measure of aggression that resulted in him inviting the celebrated Jesus to his home. He was a man so self-concerned and inhospitable that he failed to offer Jesus even the customary services a host provided visitors to his home. Simon probably felt himself at least Jesus' equal, and his conclusion that He was no prophet perhaps indicates that he styled himself as Jesus' superior. He likely considered Jesus nothing but an interesting celebrity who could gain him recognition in the community for having Him as his guest.
His evaluation of himself in relation to Jesus produced in him no sense of obligation, and thus no gratitude, humility, or act of love, let alone common courtesy. Had he a heart at all? He was scandalized by this dramatic and arresting scene taking place at his respectable table.
While God considered her act of love to be so awesome that He had it memorialized as an eternal witness, Simon's perception of it only concluded, "She is a sinner." No, Simon, she was a sinner, and therein is a major clue to the reason for their differing reactions to Jesus. In Jesus' parable, Simon and the woman held something in common—something Simon did not grasp, but the woman did. Both were debtors to the same Creditor, and neither could meet their obligations, but Simon did not even see his indebtedness.