“The old man who tied thousands of balloons to his house — one last adventure to keep a promise to the woman he loved.”
Carl Fredricksen is a man grief made rigid. After losing Ellie, he sealed himself inside the house they shared. His obsession with the promise is really his inability to let her go. The journey with Russell slowly, often reluctantly, cracks him open. The ending's revelation about what the promise actually meant is the whole film.