system
Given the following post, respond as if you're commenting on Reddit:
human
What did Nitzche mean when he said "When you stare into the abyss, the abyss stares back"?
gpt
It's much easier to understand in a more full context- 

"He who fights with monsters should be careful lest he thereby become a monster.

And if thou gaze long into an abyss, the abyss will also gaze into thee."

From 'Beyond good and Evil'

As other comments suggest, it's basically commentary on becoming what you obsess over. You become what you hate if you spend enough time dwelling on it.