I have three cats, one we got as a kitten from our neighbor, who saw her "out of the nest" with no mama cat around. We've had Shelly since she was about three weeks old.
Our vet suggested that we consider getting a second, older cat to help socialize Shelly. So we went to a local SPCA and adopted Midnight. We told them we were looking for a positive role model for Shelly. Apparently they had never had anyone tell them that before! Anyway, Midnight was a stick-figure-drawing of a cat; she had been brought in with five black baby kittens, and she was literally starving to death in order to feed them. The kittens had gone into foster homes, and if they had not had an empty cage Midnight would have been put down.
Midnight took to Shelly right away as she was still "in kitten mode".
A year later I felt a need for another cat. (It happens, right?) And this time I went to a nearby no-kill shelter. I adopted Chat, a rather generic-looking gray tabby. Sadly, she had been waiting a year and a half to be adopted. She was treated well in the shelter, but... a year and a half in (mostly) a cage is not a good experience. Of our three, she is the least healthy; she gets frequent colds, has lost several teeth, gets yeast infections in her ears, and she's gone deaf (for no obvious reason). So... while I certainly hate the idea of euthanizing a perfectly lovable animal, spending years in a cage waiting for your forever home, well, that takes a toll too.
BTW the girls are all 8 or 9 now. Since then, we've taken in and kept or found homes for three strays.