No, it wasn't because of any one thing, Ginko had already said that. But they weren't mistakes Ginko had made either! It was Foster stealing and Psi misunderstanding and Sans loosing his patience. They were things that involved Ginko, certainly, but they were not in any way Ginko's fault. They were just things that happened, feelings people would have regardless of the people around them or the situations they found themselves in. Psi would misunderstand, Sans would loose patience, Foster would make himself a target. They were things that happened and would continue to happen with or without Ginko present. They were nothing more than the totally normal results of interacting with another person.

The realization that Ginko doesn't understand that happens suddenly. Psi doesn't know how to explain it either, and he knows that immediately. Like adding vinegar to baking soda, his internal reaction is dramatic and instantaneous. It's a feeling he doesn't know how to explain. It's not frustration, guilt, or shame. It's not any of the powerful emotions that normally robbed him of his senses. It was sorrow, sadness without anger, intent without impetus. There's an uncomfortable sense of loss alongside an uncomfortable sense of acceptance and he realizes that not only did he not know how to help, it was unlikely that he could.

He pulls back from Ginko, studying the human intently. What could he say? What should he say? In the end, it's what he wants to say that moves him.

"I'm sorry."