Say all the things you've been holding back all day.
Sometimes, you just want to yell. Lots of frustrating things happen to you in a day, and you have to keep your cool regardless. But when you get home, your pet is always down to listen.
You can vent to them about what you SHOULD have said to the man who was rude to you on the subway. You'd probably never actually say that in public, but it's nice to be able to get it out to someone who won't judge you for it.
There are days when you're not sick, exactly ... but you really, REALLY do not want to go to work. And so, you claim sick.
If work asks? You're sick. If friends ask? Yeah, you're not feeling great. If your parents ask? You're sick but they really don't need to panic - you promise. If your pet asks ... OK no, you're not sick. You just want to marathon Netflix all day long. "Sick" days where you stay home and cuddle are your pet's favorite days, anyway.