Betty really wanted to finally be able to give her son the puppy that he had always wanted, but she knew that Andy would soon bore of the puppy, as it grew bigger and then the responsibility would fall on her and her husband to look after the dog.
Andy was far too involved in video games and such to be responsible enough for a pet such as a dog.
A cat was so much easier.
And of course, her husband, Cameron, would never allow a dog in the house.
Especially after what happened to him, as a child.