I took it as revenge. Joseph is toying with his brothers in order to get even with the fact they had sold him into slavery. The KMTT podcast tells me otherwise. The brothers arrive in Egypt because of the famine in Canaan. The viceroy (Joseph) send them back with allegations and to return with their brother Benjamin. Then he frames them to appear having stolen the viceroy's cup and they are to go back to the palace once more. He threatens to take Benjamin.
That is when Judah steps forward (parashat vayigash) with his plead and Joseph reveals himself. Why does he stop his game here and now? Enough with the revenge, I always thought; and now he wants to know whether father Jacob still lives. But Rabbi Waxman claims Joseph was on to something else altogether and he abandons that strive in vayigash. Joseph was trying to make his childhood dreams come true. He had had his brothers bow in front of him, which resembles the dream of the bowing sheaves. Now he wants to make the next dream come true, when the eleven stars and the sun and the moon bow to the twelfth star.
But he cannot go on and thus he gives up on the dreams of grandeur and opts for another interpretation of the dream of the sheaves: this was not about him ruling his brothers, but rather providing for them. He takes the selling into slavery as God's providence; sending him ahead and acquire the position he did. All of this allowed him to be able to provide for his family, once famine struck.