A friend of mine recently had her dog euthanized because of brain encephalitis. It was due to an adverse reaction to the vaccination she had a year ago (back when she was just a puppy).
They tried everything, but her behavior had gotten worse. She kept on attacking her packmates and was getting even snappier. She would often be gentle then a little later like a wild aggressive dog.
They went to the vet and asked what else they would be able to do but that was what the vet answered because the dog had onset schizophrenia. After trying a number of treatments and homeopathic remedies for a year, there wasn't anything else that could help her so they decided to have her euthanized.
Have you ever had to euthanize a dog due to an ailment or old age?