system
Given the following post, respond as if you're commenting on Reddit:
human
Eli5: Why do humans develop mental illness? Hoe come animals don't seem to do develop them like we do?
gpt
Short answer: some animals definitely have a mental illness.

Less short answer: we aren't psychic and they can't tell us, so some animals seem to and probably do have something we would consider things like depression, and anxiety disorders. We just don't humanize them the same way, so we just call the animal anxious, excitable, etc. 

More detailed: our conception of the inside of an animals mind is constantly changing. It was only in the last few decades that people generally even believed that animals like *cats and dogs* were even capable of feeling pain. People genuinely thought that all feelings of any kind were part and parcel of the human domain.

As we learn more, we of course learned that all higher mammals at the VERY least have physical sensations more or less identical to what humans experience. They just express it differently. Perhaps animals with FAR less developed brains may feel things like pain as well. This is an area of current research.

In terms of emotions, also recognize that only *in the last several decades* has something like mental illness been recognized in the modern sense that you probably mean it. Long ago, people thought they were demons and whatnot. Not as long ago, some mental illnesses were not even recognized as something that even existed. They were attributed to moral or ethical failing, or being weak, or being something other than actually ill.

With all this in mind, it is clear that some animals definitely feel things that we would call depression. Social disorders exist in all higher animals. Emotional ones too.

But, throwing all THAT aside there is definitely a connection between modern living and greater amounts of depression. Modern pressures are more acute and lingering than they ever have been before. While we spend less time physically sustaining our existence with labor, we spend WAY more time being occupied by things like work. We have less free time, we have less socializing time, we have fewer outlets for our problems. 

Most animals get plenty of outlets for all those things. But, keep an animal cooped up and bored and you'll quickly find that it behaves just like a depressed person would.