Deuteronomy 32:18 can be expounded as follows:
“You neglected the Rock that begot you” – the Rock, God, created you with the capacity to forget, so that you should not remember everything that happens to you.
We experience trouble, pain, and suffering. God was concerned that all this would weigh down on us, so He created in us the ability to forget.
Yet what did people do?
The verse concludes, “forgot the Lord who brought you forth.” God created you with the capacity to forget things that you don’t need to remember, but instead you forget God Himself.
–Rabbi Adin Steinsaltz
From Talks on the Parasha