Reason

Erwin always felt that his eyes were a bit unique compared to those of other humans. He didn't know whether he was born with them or trained them subconsciously. 

However, his eyes certainly could pick up more details from everything. Even for a human, he could see not only their appearances but also the way they behaved. He studied psychology so that he could fully utilize his eyes.

But never in his wildest dream would a certain quote appear in his mind.

'If you stare into the Abyss long enough, the Abyss stares back at you.'

For a person who stripped people's bare with his eyes to learn more about them, the opposite party would also stare at him. But this time, the one who was stripped naked was him.