פירוש על עבודה זרה 10:1

The Israelites in the desert were ungrateful, refusing to recognize the good that God had done for them. They were the descendants of Adam who was ungrateful to God for the woman whom he had been given.
Only at the end of their time in the desert did Moses tell the Israelites that they were ungrateful.
Rabbah learns from the experience of the Israelites that it takes forty years to learn the mind of one’s teacher. So students and teachers, give it some time!