pets can't talk, people can. Its pretty simple.
I disagree with that.
The problem with humans is our arrogance. We expect everything to communicate the way we do (to talk). Some animals have a very complex methodology of communication (elephants, wolves) that not only involve vocalizations, but also body language. They just don't have the mimicry ability to communicate in a method humans recognize. Hell if anything, you can consider most "pets" that you can train to be bilingual. They speak their species language and they speak whatever language the humans they interact with.
If you know an animal and you watch it, they will tell you if they are sick. You just have to pay attention and listen to them.