MAJOR SPOILER WARNING

The ending is a bit vague to allow the viewer to interpret it their own way, a lot of movies do this kind of thing because it sparks discussion and interest around the film.

My interpretation is that Robot had a change of heart when they thought about what would happen if they and Dog would meet up again. We get to see what Robot thinks might happen just before they play "September" for the last time in the movie. While the time Robot and Dog had together was a delight to them both, they spent an order of magnitude longer separated from each other. As the seasons passed, things changed. While the film was largely driven by both the main characters desire to re-unite, they ended up moving on instead. I think that's what makes this ending so special. It could've been a cliché fairytale Hollywood ending, but I don't think that would've earned it an Oscar nomination. Instead, we get a bittersweet dose of reality, and it's not easy to swallow.

Dog has his new friend, Palbot. They're getting along great, and Dog has grown as a person and friend. Robot has kindled a new friendship with Rascal, who they kind of owe their life to. If it weren't for Rascal, the movie would've leaned over into a pretty devastating ending where the only takeaway is how cruel everyone was. Instead, Rascal gets a new friend and someone who can help him do his job. In the sequence where Robot imagines his and Dog's re-unification, we see Palbot off to the side, looking quite sad. They're likely thinking that they were just a "replacement" to Dog. That's why Robot can't just go right back to Dog and how things were. They don't want to barge in and ruin things between Dog and Palbot. Not to mention they wouldn't want Rascal to think they're ungrateful for what he did for Robot. So, the movie ends with a bittersweet musical send-off.

All of this is not to say that my interpretation is the correct one, that's the beauty of open endings like I mentioned before. There is no right answer. I haven't read the graphic novel this movie is based on, so there may be more to decipher there. However, considering the movie was co-written by the author of that novel, I'm not so sure. With my interpretation, there's also no reason they couldn't re-unite later on, when the time feels right. Robot remembered what Dog's neighborhood looks like, so they kind of know where he lives.

I think the lesson is that real life is often more complicated than in stories. That it's wise to consider the repercussions of following our most immediate desires and instincts. I've watched this film multiple times and find the ambiguity of the ending to be a huge pull.

To directly answer the question of "Why not just switch and go to dog and new robot can go to rascal?": I don't think that would work out so well. Neither Rascal or Palbot know each other, and they would both probably rather be with the friend they've already made.

Thank you Levi for your well-articulated perspective on the ending, the meaning of the movie as a whole and for making me love it even more.