He has spent his lifetime behind the keys, pouring everything he knew into a single monumental opus.
In the present he keeps the peace between his son and daughter, and between his cat and dog. Although the cat has spent more effort on guiding his writing, it still the dog he thinks of when someone mentions a loyal pet. The cat just shrugs, as there is only so much you can do to guide a stubborn human. The son and daughter are both wise enough not to take sides.
As his final opus nears completion, he almost fears the moment when it’s creation will all be in the past, and there is nothing more to add. A life’s work will be the sole legacy of how he spent his life. The son and daughter moved on to their own lives, and the cat and dog continuing their quiet rivalry.