In an early interaction, God told Moses that He will send his angel to guide Israel along the way. The people of Israel were warned not to disobey God’s messenger, because he would not forgive their transgressions. (Ex. 23:21)
Moses approached God with a very bold request indeed; He asked for God to personally accompany Israel instead, refusing to move anywhere without His own personal presence. Why did Moses take the risk of challenging God? Why did he think that the original arrangement would not work?
In the end of incredible experience of seeing the back of God, hearing the words that described his fundamentally gracious and forgiving nature (Ex.34:6), Moses disclosed his real reason: "If I have gained Your favor… let the Adonai go in our midst, because this is a stiff-necked people. Pardon our iniquity and our sin, and take us for Your own!" (Ex.34:9)
In other words, God should go with Israel and forgive them, precisely because they are stiff-necked people! Since God already said that angel would not forgive Israel if they rebel against him (Ex.23:21), Moses knew that his only hope was to persuade God Himself to come instead. While being hidden in the cleft of the Rock, Moses understood that YHWH (unlike His angel) was able to forgive “iniquity, transgression, and sin” (Ex.34:7). It was a match made in heaven: Israel had sins, YHWH had forgiveness.