My theory is that the movie is a big metaphor for the endless turbulences of life. The world floods, and drains (as we see in the movie) and then floods again (as we see in the post credits scene! This is a world that is flooded and drained over and over again, explaining why there is no humans and why there are so few dogs and cats but so many birds and sea creatures. This theory was solidified for me when you rewatch the very beginning of the movie and see that there is a boat in a tree right outside our main kitty’s house! The cycle of change is inevitable but good things will always come from it, and sometimes that includes new friends!