And they did not recognize him...
The sons of Yakov have all chosen to be shepherds, a quiet and peaceful occupation. Out in the fields, tending their flocks, they had little contact with the social life of the country and were undisturbed in their service of G-d. The brothers of Yosef felt it necessary to select an occupation that would help them lead their G-d fearing life. They did not wish to live in an environment that would place temptations in their chosen path. Yosef however, was in this respect superior to them, he was able to have the highest administrative position in the mightiest nation and yet still remain righteous.
The brothers could not recognize and could not comprehend how the ruler of Egypt could be the same G-d fearing Yosef, whom they had known, for such a way of life was above their level. In fact, also Yakov, Yosef's own father was a bit apprehensive and was worried when he heard that, "Yosef was still alive and was the ruler over the land of Egypt", he was scared that perhaps he had assimilated into the Egyptian culture, G-d forbid.
When his sons explained to him that although Yosef was ruler of Egypt but he still conducted himself as is befitting for a son of Yakov, was Yakov satisfied and able to experience true joy.