Maybe separate subsystems in the mind had to evolve to follow the same leader ("I") as obediently as possible because pausing to debate with other subsystems at some critical moment instead of running meant that a predator had time to kill you. The unification of subsystems from this pressure would tighten over time, maybe to the point where a subsystem challenging the primary conscious agent would be perceived as mutiny by the other subsystems and they would try to bully it back into submission by producing anxiety. I wonder if a subsystem successfully powering through the protest and claiming the throne of "I" is what happens when people snap.