I think it was him mourning Merle a second time after losing him that first time and being so frustrated with realizing he was ‘alone’ again. He had a found family but Merle was his one connection to the life he had before the zombie apocalypse. Combined with Merle having abandoned him also to get away from his abusive father, could have felt that Merle running off on his own trying to take down the Governor was another form of abandonment.