system
Given the following post, respond as if you're commenting on Reddit:
human
how do certain animals know to look us in the eye?

I have always noticed that when speaking to a pet, whether it is a dog or a cat, they somehow know to make eye contact with me as I am doing so... I always thought this was a social human interaction. How do they know to do this? Or is it common with other animals.
gpt
The eyes are extremely commonly used in body language across most land vertebrates.  

Focusing on the eyes tells you what an animal is paying attention to and gives you some insight in to its emotional/arousal state, because the biochemical changes inherent in arousal lead to different changes in the eyes.

From there, animals that use body language to communicate will tend to build upon pre-existing foundations, which is why facial expressions/movements are common in mammals and, to a lesser extent, birds.