Got your tissues handy? Good, because while this is ultimately a good story, that doesn't mean it isn't a sad one.
Animal shelters are used to finding unwanted pets left at their door. There's never really a good reason for a pet to be abandoned. Sometimes a family simply can't afford to keep them, or they don't get along with a new baby or spouse, or maybe the owners were just over having a pet. It's always sad for volunteers to find a box by the door.