The Rabbis explain that the reason for the exile in Egypt was to forge the individual descendants of Jacob into a nation. That is one of the reasons that Egypt is described as being a "melting pot" - that it took the disparate parts and turned them into a nation that could stand at Sinai and receive the Torah from God. The style of the exile, however, was dependant entirely on the Jewish People. If we would have remained true to the teachings of Abraham, Isaac and Jacob the exile would have been completely benign. There would have been no physical slavery or affliction - it would have been completely spiritual and mental in nature. Whatever needed to happen in Egypt would have happened and we would have left after four hundred years without having to go into exile again. The Sages explain that because the Jews started to assimilate the exile became a physical one as they had lost sight of the spiritual dimension of their exile it became clear that they would have to suffer an exile that they could understand.
