"O Allah! Make me among those who, when they commit an act of virtue, feel good, and when they commit a mistake, they seek forgiveness." [Ibn Majah]
Tawbah (repentance) and Istighfar (seeking forgiveness from Allah) are among the most meritorious acts of virtue for the believers.
The act for which we perform tawbah and istighfar is not necessarily a sin, or an act of disobedience to Allah; it also includes our shortcomings. As we realize Allah's immense favors to us, all of our thankfulness and devotion clearly appears to be inadequate. As we realize the grandeur of Allah, Most High, our acts of worship and obedience clearly are seen to be insufficient. The higher a person is on the scale of taqwa, piety and God consciousness, the greater is this sense of inadequacy. Consequently the greater is his/her practice of istighfar.
That is why all the Prophets (as) preached and practiced tawbah and istighfar. We do not have to invent any sins, inherited or personal, to explain their repentance. In fact all the prophets were free of sin, as Allah appointed them as role models for humanity and Allah did not send defective role models.
The leader of all the prophets was Prophet Muhammad (saw) a fact which was also symbolized in his leading of all the prophets in Salah (prayer) in Jerusalem during Isra'. And what did the prayer leader of the prophets use to do after every prayer? He used to say astaghfirullah (I seek Allah's forgiveness) three times! This is the istighfar that comes out of the highest level of God consciousness! He taught us to perform istighfar profusely, as he himself practiced.
Istighfar is also a means of enhancing that consciousness of Allah and strengthening our relationship with Him. We turn to no one except Allah in repentance. We confess our deepest errors, shortcomings, failures, and sins to Him and Him alone.
We need istighfar to constantly purify and cleanse our heart. We are not born in sin, but we are born in weakness. We are prone to fall prey to the many temptations that are part of our test in this life. And when we do fall and commit a sin, it produces a dark spot in our heart. A famous hadith, reported by Abu Huraira (ra) describes this process. When a person shows remorse and repents, that dark spot is removed. Otherwise it will stay there and grow with each additional sin. A time may come when his heart is full of darkness because of un-repented sins.
Today, unfortunately, we see so many examples of this all around us. In the "everything goes" post-modern world, good and evil do not mean anything anymore. Then there is a whole crop of misguided psychologists who are ready to assure you that the only guilt you should feel is for feeling guilty in the first place! Is it any wonder that in the English language the word sin is now normally used to describe things that are delicious, attractive, fun, and highly desirable? That this darkness of the heart should be considered enlightenment only completes the inversion.
But there is hope for those who seek hope. No matter how corrupt we might have become, we can always make a U-turn. We can repent and seek forgiveness from our Beneficent and Merciful Creator Who is always ready to forgive those who turn to Him in sincerity. "O my Servants who have transgressed against their souls! Despair not of the Mercy of Allah. Verily, Allah forgives all sins: for He is Oft-Forgiving, Most Merciful" [az-Zumar 39:53]. Further a hadith declares:
(courtesy of http://www.albalagh.net)