When I talk to pet parents before the loss of their pet, they frequently mention that they are waiting to move forward with euthanasia because their pet "isn't in any pain." Since our pets cannot verbally express themselves, pain is often defined by the family as when the pet is whining or crying.
So many pets do not express pain vocally, instead they hide or reject our efforts to cuddle and comfort them. Pet parents who are waiting for whining and crying as the "sign" that their pets are in pain, may want to think of misery or suffering as the barometer that leads us to make a decision to move forward with euthanasia
Think of when you yourself were miserable with an illness, and while you may not have experienced "pain", the misery was awful. Were you restless and inconsolable?
When you have consulted with your trusted veterinary professional, and you have a reasonable expectation that there is no treatment that can turn your pet's health around, define the condition you are evaluating and trying to prevent for your beloved pet as "misery".
When that time comes, saying goodbye to your beloved pet releases them from misery, giving them the most loving and compassionate gift of love.