One of the criteria for the existence and operation of the ego is: 'who I think I am;' more accurately 'who my mind thinks I am.'
These thoughts come together to create an image in the mind of one's identity. Expectations, the result of exposure and experience, desires, values, and other related components, also contribute to the mind's identity image.
Over time, our mind uses this ever-changing image as a blueprint to run our lives. This is one of the root causes of suffering. The discrepancy and lack of harmony between who one really is versus the mind's image automatically leads to an imbalanced life full of contradictions and pain. There are inevitably some happy moments too now and then, but nothing in line with our potential for truly enjoying this Universe.
This usurping of identity also leads to never-ending karma if unchecked.