I have worked in a shelter and also have had to euthanize many animals- most of the ones I euthanized had a health, behaviour, or age related reason. I did feel awful for the geriatic pets that got dropped off by their owners who had convinced themselves that someone else would want to adopt their aged pet. However, today I spayed my first shelter cat and she is going to get adopted soon. I'm glad she won't be having kittens.
z.