The tragedy of betrayal deepens significantly when it is paired with the sin of forgetting. Deeply immersed in idolatry and improper acts, the Israelites completely erased the memory of their miserable and vulnerable beginnings, ignoring the kindness of God who saved them from the lowest depths.
The primary approach among commentators is that the nation compounded its guilt. Along with their unfaithfulness and the act of giving away all they had to others, they added the ultimate failure of forgetting their past [מצודת דוד, רד״ק, ביאור שטיינזלץ]. Another perspective suggests that these very wrongdoings are what caused the complete memory loss. By engaging in such acts, the people forgot God entirely, choosing instead to believe their survival was the result of blind chance [מלבי״ם].
The central rebuke focuses on their failure to remember the days of their youth, a time symbolizing the period when the Israelites were enslaved in Egypt. They forgot the immense kindness and the miracles God performed when He freed them from harsh labor [רש״י, מצודת דוד]. Had the nation remembered its history, the people would have feared God's punishment and the terrifying possibility that He could return them to that exact state of slavery [מלבי״ם].
This difficult past is depicted as a time when the nation was entirely destitute and unclothed, rolling and trampled in the filth of its own birth [מצודת דוד]. Their early vulnerability is presented as a collective whole, capturing the many harsh realities and sights of their difficult birth as a nation [רד״ק].
A deeper layer of this relationship is revealed through a subtle contrast in how the message was recorded. While the spoken message is directed at the nation for forgetting its past [מנחת שי, רד״ק], the written text carries the first-person voice of God. This hidden layer reveals God expressing that during the days of their youth, He actively chose not to look ahead at the sins they would eventually commit, having mercy on them despite knowing the future [רד״ק].