I had a cat - Nigel, his name was. Big dark tiger tabby, he was a really good pet. Even though he had temperament issues (biting people when they petted him anywhere other than on his head, exerting dominion on every other cat in the house) he was really a nice cat. He had no problems using the litter box, was not terribly vocal (except at feeding time: god, he made some noise), and overall behaved himself.
A few years back - four or five - he started to lose weight. He was at an overweight 17 pounds at the prime of his life: I saw him go from that to fourteen, then to twelve, eleven, and then nine. He was literally wasting away: he wouldn't eat, he puked constantly, he seemed under-the-weather, down, and would constantly seek reassurance. He was beginning to do his duty on the carpet: when he got down to seven pounds, my family decided it was getting too bad.
I had left on a trip: one night a few weeks after, my family found him in my room, hiding under my bed and puking everywhere. They decided to take him to the veteranarian's office and see if they could do anything. It turns out they couldn't: although they could temporarily relieve the problem, it would come back in about a month and it was pure torture for him. So my mother did the sensible thing: put him out of his misery and had the doctor give him a euthanasia shot. He slowly passed away in my mother's arms, while she cradled him, crying miserably.
Nigel suffered from a liver failure: his liver simply stopped working altogether. While it was sad for all of us, he was really better off just fading away. I didn't hear about his death until I got back from my trip, but even then I had expected it that the minute I walked out of that door to go was the last time I ever would see Nigel. I made sure to stroke him a few times and give him a headscratch - Nigel's favorite.
The only night I did cry for him, in fact (a few months after I had returned), I went to sleep fitfully. I had a lucid dream that night - my very first - where Nigel was once again sitting on my couch, fully healthy once more. I went up to him, knowing he was dead and this was a dream, and stroked him a few times. A little bit later, he jumped off the couch and slowly walked across my living room, tail straight up in the air. He proceeded to sit down on another couch on my family room (a plaid, beat-down couch - his favorite, once again) and slowly curled up into a ball. I hugged him, chanting "Thank you, Nigel," two times, before waking up crying once more.
It remains to this day the best dream I've ever had.