Relic of the Future: Chapter 139

Pietro stopped wringing his hands together. "He filled me in on the details of why it's not a good idea; how Grimm are drawn to those with their aura unlocked. He couldn't say why that was, but after days of research and reading, I've come to realise he simply doesn't know. No one does. The leading theory just seems to be that Grimm hunt negativity and those with their aura unlocked project that more keenly. Perhaps emotion comes from the soul and having it coursing through your body makes all emotion, good or bad, more visible to the Grimm. It's hard to say for sure."

—Relic of the Future: Chapter 139