Part of the etiquette of du’a is to acknowledge our shortcomings and sins in front of our Creator. This is part of the means of properly worshipping Allah.
Ali ibn Abi Talib reported that the Prophet, peace and blessings of Allah be upon him, said:
“Verily, Allah likes a worshipper who says: ‘There is no one worthy of worship except You. I have wronged myself, so forgive my sins, for none forgives sins except You.’ Allah says, ‘My servant knows that he has a Lord who forgives and punishes!” (Recorded in Ahmad)
This is why the best du’a is the one in which a person asks for forgiveness and sincerely acknowledges their deficiencies and sins. The Prophet, peace and blessings of Allah be upon him, said:
“The sayyid al-istighfar (the best du’a to ask for forgiveness) is that a person says: O Allah! You are my Lord, there is no one worthy of worship except You. You created me, and I am Your slave. And I am (following) Your covenant and Promise as much as I can. I seek Your refuge from the evil that I have done. I acknowledge Your favour upon me, and I acknowledge my sins. So forgive me, since no one forgives sins except You!” (Recorded in al-Bukhari)
When we raise our hands to Allah, expecting His response, we should also ponder our own relationship with Him, and ask ourselves about the shortcomings that we possess. We are reminded that we are calling on the One that we have disobeyed, and sinned against, and we are asking Him to bestow upon us favours, without meeting our own obligations. In this way, when we recall our many sins, it creates a sense of humility before our Creator.