"But, he lived it too..." It's a discouraging thought. It isn't Finn's fault that he's the one who stayed in human shape, and there's nothing he can do to give that existence back to Fern.

"Fern is Fern now. And I think that's great! It's like we're really brothers. I'm sure everyone else would like him too if he gave them a chance..." And now Finn is stuck here, and Fern is back in Ooo upset about something they can't change and Finn can't go back and help make it better. Maybe he's even made it worse before without realizing. The butterflies all start to land, covering his hat and shoulders. He ignores them.