Some people are not capable of thinking about what they do in life could affect other people and that they do have responsibilities.
Also, why is everyone saying put the dog down? I have arthritis and severe haemophilia, on a bad day I can't get out of bed and no amount of painkillers helps. I would not however want to be put down, especially not with the family I have and the thought of having to leave them to cope with my selfishness.
Some people need to learn that their pets lives are not theirs to end. Something like arthritis is not a good reason to put a dog down, at a point like that you need to sort yourself out and look after the dog a bit more, spend some money on treatment and accept the fact that you took on the dog and agreed to look after it till it does eventually die. Not just throw it away the moment it becomes an inconvenience.