Disclaimer: I don't own Megami Kouhosei.
Reason
Ernest was not a violent person. He also greatly disliked physical contact with others. Ernest was the kind of person who looked at things with serene eyes, thoughtful and thorough, always a mediator.
There really was no explanation for having punched Gareas, then.
He hadn't wanted to think or to calm down this time, to be rational and keep the peace. Ernest had had a point to prove. His only thoughts: he couldn't afford to lose someone like Gareas, no matter how hard Gar would struggle to tear away from him, from everyone. Gareas didn't have his own wings, and he could fly. But he didn't know that.
Ernest refused to release the hold he had taken in this person, this only person who had seen his thoughts, this only person he had offered his hand to, the one he had let touch him, fingers straying. Gareas didn't touch him anymore, too focused on fighting on trying on surviving, and Ernest didn't share so many thoughts with him anymore, just like everyone else he didn't share his thoughts with, but the fact was that he had. Ernest had seen the inside of Gareas' head, and even if it would never happen again, even if Gareas had yanked free too much of his own leash to be close enough for the tips of their fingers to brush anymore, Gareas had been the only one. Gareas would stay because of it.
Because Ernest can break too, can need too, and sometimes he can fight, too.