Conceit causes one to forget many sins which he committed in the claim that he does not need to reform himself, so he does not mend what he had broken. As a result for this heedlessness, he forgets many sins, and he is not concerned about the sins which he still remembers. This condition may drag one into committing new sins.
Perhaps this is the meaning referred to in a tradition quoted in Wasāil al-Shī'a from Imām al-Sādiq (ع) who cites the Messenger of Allāh (ص) in a tradition in which Mousa (Moses) son of Imran (Amram) asks Satan, "Tell me about the sin which, when a son of Adam commits, you overtake him." Satan said, "It is when he is proud of himself, thinks too much of his good deeds and his sin looks small in his eyes."
It is known that when Satan overtakes someone, the result will be more sinning, in addition to underestimating the sin, for this by itself is an insult to the status of the divine greatness which is one of the greatest sins, and it may stop one from being included in the divine mercy, as some traditions point out. In the sacred book Al-Kāfi where Zaid the cooking oil seller cites Imām al-Sādiq (ع) saying that Abū Abdullāh said that he had heard Abū Muhammad (ع) saying, "One of the sins which are not forgiven is when a man says, 'Blessed am I had I only not done anything other than this [deed]!" Abū Hashim al-Ja`fari quotes Imām al-Hassan al-`Askari (ع) saying that he heard Abū Muhammad (ع) saying, "Among the sins that are not forgiven is one saying, 'I wish I would not be held accountable except for only this [deed]!"
Imām al-Khomeini, the mentor, may I be his sacrifice, has explained that conceit obliterates belief and ruins the rewards for the good deeds, according to a tradition narrated by Ali ibn Suwaid. The imām was asked about the conceit that spoils good deeds, so the imām explained some of its degrees, citing other traditions as well, in this regard. He said the following:
"The conceit tree is bad; its fruit is many major sins and abominations. When its root rests in the heart, one's affair is dragged into disbelief and shirk and more than both of these. One of its consequences is underestimating sins; rather, one who is conceited is not in the process of reforming himself claims it is pure and purified. He does not care at any time to purge himself of the filth of committing offenses. The thick curtain of conceit blocks him from seeing his own ills. This catastrophe stops one from achieving perfections, afflicts him with all sorts of shortcomings, brings about eternal perdition, and the doctors of the psyche can find no cure for him."