Rick doesn’t have an answer to that question. The version of him we see in the episode’s final moments is completely quiet, lost, and disconnected from the world around him. Everyone else in the Smith household seems to be having a great time, but Rick’s incapable of staying present. He’s too wrapped up in what just happened, seemingly incapable of coping with the idea that he might not have any purpose left.
Throughout this montage, we hear Mazzy Star’s song, “Look On Down From the Bridge,” with all the characters’ dialogue cut silent. The result is a scene that serves as a blunt parallel to the season 1 episode, “Rick Potion #9,” where a stunned Morty walks around his new home, shell-shocked, still struggling to accept the massive events of that week’s episode.
Morty’s reasons for being upset back then were pretty straightforward. The episode had him accidentally kill nearly everyone in his original reality, so he and Rick had to go to a new reality and bury the corpses of their original selves. It would be a traumatizing experience for anyone; the fact that everything goes back to normal afterward might seem like it’d help, but all it does is reinforce how meaningless his world is. If the horrors Morty’s witnessed throughout the episode could be so easily swept under the rug and left behind them, if everyone Morty’s ever known can be replaced with near-exact copies that easily, then what’s the point of anything? Rick’s often maintained throughout the series that nothing matters, but this is the first time Morty has truly been forced to take that message to heart.