I think for us, with Buster, it was seeing how fast he deteriorated. It's barely been a month since we started really noticing there was anything wrong with him, starting with how he wasn't really grooming himself: he was a Persian but kept himself quite well groomed, so when the knots started getting noticeable it was clear something was off. Then about three weeks ago my wife noticed a big lump on his head, just above his right eye. We were concerned but thought it might have just been from getting nipped by the dog or something.
A week later, we noticed his right eye seemed to be bulging out, and he was really slowing down in getting around the house, and we realized he was no longer jumping up on the bed for skritches after we'd tucked in.
We took him to the vet, and she said it seemed like it may be a tumor, but could also have been an abscess or infection. She gave us some antibiotics for him and said to come back in a week for a follow-up. She also noted that they could do surgery to remove the lump, but that could cost several thousand dollars and if it was in fact a tumor, there'd be little else they could do for him.
Two days later we saw that he could barely make it down the stairs; my wife noticed that his right eye didn't move to follow her the way the left eye did, and then she watched him squat and pee in the middle of the floor and then just sit in the puddle.
It was that point that we decided to book him for a final visit that evening. When we got home from work, we spent some time giving him a bit of his favorite Friskies, giving him his ear and head skritches, and just showing him he was loved... and then off to the vet to release him from his pain.
That was about a week ago. It's still tough, but we know it was time for him. He was 11, still just middle-aged for a cat, but he had a good run.
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