We spoil our pets so much these days that most of us thing they have nothing to complain about but, much like humans, they almost always have some little complaints.  According to animal psychics, the most common complaints are:

• Cats complain about the litter box; not necessarily the cleanliness, but also location, size, height, etc.  The next most common complaint from cats is boring food
• Dogs primary complaint is that their people don't spend enough time with them or don't spend time playing with them when they are home
• Birds complain about people constantly sticking hands in their cages and cages being cluttered with too many toys (apparently, birds like a spartan, but roomy environment)
• Horses like things to happen on schedule and get upset with people who can't maintain a proper schedule.

Hopefully, that information will give you some insight into your own pets.  Of course, pets are individuals too so their complaints may be very different than these most common ones.