Sad Dog Diary is on point. Especially if you are in a dog + cat household.

Why is the dog sad about being in a dog and cat household?

The dog's not sad about being in a dog + cat household, the narrator says some "the cat said" bits where if you had the two, would totally work to make a dog sad because most cats are usually jerks to the dogs they live with.

Some cats are jerks to the people they live with, too.