The fast is a source of forgiveness for one's sins. Hence, it removes the effects and stain of these sins from one's heart, leaving the heart pure again. The Prophet (peace be upon him) said,
"Whoever fasts the month of Ramadan with faith and hoping for its reward (ihtisab) shall have all of his previous sins forgiven for him." [Bukhari, Muslim]
Al-Khataabi notes that the meaning of ihtisab means with resolve, hoping for its reward while being pleased to perform it without considering it something heavy or a hardship. Hence, the true believer for whom the fasting will have its desired effect of removing or effacing his previous sins approaches the fast with a desire and love for it. He does not consider it a burden nor does he participate in it lackadaisically, with some heavy burden on his shoulder, being lazy and complaining until it is time to break the fast.
"Purification of the Soul: Concept, Process and Means" - Jamaal al-Din M. Zarabozo, pp. 239, 240