Vayeshev - Cycles of Growth

What is the reason for the oppresors of the Jewish people having power over us? What is the fundamental mistake we make in our relationship with Hashem? How do the challenges of our oppresors create a change that brings us back into relationship with Him? How does this correspond to the cycle of growth that the Jewish people experienced in the process of the Exodus from Egypt? How does this also correspond to the process Yosef went through with his relationship with his brothers?