The ego is formed and becomes the seat of power to manipulate your life because it itself, the ego, is the consequence of circumstance and circumstance is the power that sits in the seat.
The events of your life from moment to moment are the circumstances, the consequences are the results of your reactions that forms your ego. If you get cheated in business, you may get bitter towards business or if you get a terrible heartbreak, you may become closed emotionally.
Circumstances are therefore how your ego forms into who you are.
The defence against this is the will power not to be subject to the consequences of circumstance.
To gain will power we do things differently.
If circumstances are such that everyone is cheating and stealing, then your consequences are that you may feel that you must cheat and steal or lie to survive. So you are now submitting to the will of circumstance.
But if you maintain perfect integrity then you are not submitting to the will of circumstance and your ego is no longer formed by the consequences of circumstance, but rather it is formed by your self will.
Your ego/personality is opposing circumstance and is now back in your own control rather than under the control of circumstances.
That is why the code of moral conduct or qualities of character is so important and the simple exercises are the training to gain that strength.
All religions and mystical teachings are basically codes of moral conduct.
Circumstance also tells us how to react to certain situations by observing other peoples reactions, TV, movies, music, and in this way circumstance forms future generations and cultures.
Imagine circumstance to be an individual that forms the many parts of your self and life and manipulates everyone for you will see the same circumstances fall upon many different people in many different lands.
But the reactions of each person or culture vary. How can this be? Because of the basic principles that each culture or person is raised with, which changes the way in which they deal with similar circumstances, and that changes the way their life is formed.
Many people in a group will each react differently to a particular person. Likewise, circumstance can be seen and dealt with in the same view, as an individual person. This is why different people handle the same circumstances differently.
If you can see circumstance as a god of sorts (to use a word that gives you the idea of its vast reach and ability to be everywhere with everyone at the same time), who is not in the least concerned with you as an individual, then you may be able to deal with circumstances with the same indifference.