While a vet's job is dedicated to treating disease, it not always possible to do so. We have taken oaths to alleviate the suffering of animals. This is why it is incumbent upon veterinarians to help owners make a decision to euthanize their pet. Nay, to recommend that a pet be put to sleep.
If a dog has cancer, for example, it would be my recommendation to find out what that tumor is, where it is, the possibility of surgical resection, the repercussions of surgery, the chances of recurrence, and the presence of metastasis (spread).
If I find out that a tumor has littered the poor pet's body with metastases, it would not be my recommendation to send that dog for surgery or chemo. Call me old-fashioned. The pet's, and let's not forget, the owner's suffering following the diagnosis would far likely be greater than the quality and quantity of life that that pet would have in the weeks afterward (if surgery, chemo, etc, were done). Of course the patient's well-being comes first, but does the owner's well-being and suffering not count, too?
My views in this matter are not set in stone. Pet owners must be given their options - these decisions are ultimately up to them. Veterinary oncology has come a very long way and, very importantly, pets do not typically suffer through chemo in the same way that human patients do.
That said, part of my job is helping pet owners through these very trying times. I was issued the degree, I have a strong opinion on the matter, and I feel it is incumbent on me to help pet owners make the best decision. Very often, and especially in the aforementioned case, the best decision is saying goodbye.