I think this question most likely applies to our senior dogs. In that case, there are probably two good answers to this not-so-uplifting question. The first and foremost answer to this question is that cancer is the number one reason why we end up putting older dogs to sleep. We see a lot of cancer in our senior pets, just like we see in our senior humans.
A lot of our cancers are treatable, but keep in mind that they are not curable and the medications are not as advanced enough yet as what we can see in the human medical field. I have treated many dogs for cancer throughout my career and a lot have had wonderful success but, but eventually, the cancer stops responding to treatments and/or spreads to other places.
The second reason is quality of life. We love our pets and we do not want to see them in pain or suffer. Again, we can do a great job of managing many of the conditions that senior pets face and give them wonderful lives but, for most of them we cannot reverse or cure their disease. As their conditions progress we can start to see their quality of life decline which includes; difficulty walking, loss of appetite, cognitive changes, etc. For most people, seeing a beloved pet go through this process is often the reason that they will choose to put their pet to sleep.
One of the greatest gifts we can give our senior pets is the chance to pass with dignity and without pain.