I can't find the obvious answer anywhere: Pets taste better.
Seriously, though, the difference between a pet and a slave would first have to rely on the difference between an animal and a human. I am certain that humans have a desire for freedom, and that they understand their enslavement - I kinda doubt there will be a revolution of goldfish soon ...
on a somewhat related note:
EXCERPTS FROM A DOG'S DIARY
Day number 180
8:00 am - OH BOY! DOG FOOD! MY FAVORITE!
9:30 am - OH BOY! A CAR RIDE! MY FAVORITE!
9:40 am - OH BOY! A WALK! MY FAVORITE!
10:30 am - OH BOY! A CAR RIDE! MY FAVORITE!
11:30 am - OH BOY! DOG FOOD! MY FAVORITE!
12:00 noon - OH BOY! THE KIDS! MY FAVORITE!
1:00 pm - OH BOY! THE YARD! MY FAVORITE!
4:00 pm - OH BOY! THE KIDS! MY FAVORITE!
5:00 PM - OH BOY! DOG FOOD! MY FAVORITE!
5:30 PM - OH BOY! MOM! MY FAVORITE!
Day number 181
8:00 am - OH BOY! DOG FOOD! MY FAVORITE!
9:30 am - OH BOY! A CAR RIDE! MY FAVORITE!
9:40 am - OH BOY! A WALK! MY FAVORITE!
10:30 am - OH BOY! A CAR RIDE! MY FAVORITE!
11:30 am - OH BOY! DOG FOOD! MY FAVORITE!
12:00 noon - OH BOY! THE KIDS! MY FAVORITE!
1:00 pm - OH BOY! THE YARD! MY FAVORITE!
4:00 pm - OH BOY! THE KIDS! MY FAVORITE!
5:00 PM - OH BOY! DOG FOOD! MY FAVORITE!
5:30 PM - OH BOY! MOM! MY FAVORITE!
Day number 182
9:30 am - OH BOY! A CAR RIDE! MY FAVORITE!
9:40 am - OH BOY! A WALK! MY FAVORITE!
10:30 am - OH BOY! A CAR RIDE! MY FAVORITE!
11:30 am - OH BOY! DOG FOOD! MY FAVORITE!
12:00 noon - OH BOY! THE KIDS! MY FAVORITE!
1:00 pm - OH BOY! THE YARD! MY FAVORITE!
1:30 pm - ooooooo. bath. bummer.
4:00 pm - OH BOY! THE KIDS! MY FAVORITE!
5:00 PM - OH BOY! DOG FOOD! MY FAVORITE!
5:30 PM - OH BOY! MOM! MY FAVORITE!
'
EXCERPTS FROM A CAT'S DIARY
DAY 752 - My captors continue to taunt me with bizarre little dangling objects. They dine lavishly on fresh meat, while I am forced to eat dry cereal. The only thing that keeps me going is the hope of escape, and the mild satisfaction I get from ruining the occasional piece of furniture. Tomorrow I may eat another houseplant.
DAY 761 - Today my attempt to kill my captors by weaving around their feet while they were walking almost succeeded; must try this at the top of the stairs. In an attempt to disgust and repulse these vile oppressors, I once again induced myself to vomit on their favorite chair...must try this on their bed.
DAY 765 - Decapitated a mouse and brought them the headless body, in attempt to make them aware of what I am capable of, and to try to strike fear into their hearts. They only cooed and condescended about what good little cat I was...Hmmm. Not working according to plan.
DAY 768 - I am finally aware of how sadistic they are. For no good reason I was chosen for the water torture. This time however it included a burning foamy chemical called "shampoo." What sick minds could invent such a liquid. My only consolation is the piece of thumb still stuck between my teeth.
DAY 771 - There was some sort of gathering of their accomplices. I was placed in solitary throughout the event. However, I could hear the noise and smell the foul odor of the glass tubes they call "beer". More importantly I overheard that my confinement was due to MY power of "allergies." Must learn what this is and how to use it to my advantage.
DAY 774 - I am convinced the other captives are flunkies and maybe snitches. The dog is routinely released and seems more than happy to return. He is obviously a half-wit. The bird on the other hand has got to be an informant, and speaks with them regularly. I am certain he reports my every move. Due to his current placement in the metal room his safety is assured.
But I can wait, it is only a matter of time...