We had to let our pupper go a few weeks ago. The vet had to basically sit there and watch us while we processed the fact that we were about to pay her to kill our best friend.
For us, it was the worst day we had experienced in years. For her, it was Tuesday. She had other people waiting in the room next door, and had to go from solemn to bright and cheery over the span of ten footsteps, and she has to do that every day.
Her job is often to put a very real price tag on the life of a beloved companion. "I'm sorry, but keeping him alive will be $10,000... or we can humanely put him down for much, much less."
It's grim business. Necessary, but grim.