Tommy has had enough of being caught up in wars that we could never win. Even with most of the trouble being over, Dream being in jail, and L'manburg lost, the teenager didn't want to deal with another big arch... So he left
"I understand. You heard the place you loved was in trouble, so you came back, but I don't-I just-why didn't you take me?" Here it was, at last. Catharsis, or something close to it. "I would have hunted them down with you, Philza, the people who did that to your town. I would have given you your vengeance on a silver...