Causes of depression in cats
The owner's emotions also play a role. If the owner's emotions fluctuate, creating a tense atmosphere, or if they take their anger out on the cat, it will definitely affect the cat's emotions. Sensitive cats will completely lose their sense of security, gradually increasing their psychological defensiveness, which can lead to psychological imbalances. 
Leaving a familiar environment can cause stress for a cat. This insecurity can easily lead to depression. Alternatively, a cat may dislike its new environment and become depressed if it stays there for an extended period.
Cats consider the humans and other animals they live with as family. The sudden disappearance of a familiar member, whether they move away or die, is a huge blow to a cat. They will constantly try to find and call for them. If their owners don't understand and comfort them, this stress can become so great that it can lead to depression.
Cats become particularly frustrated when their owners' attention decreases. Cats crave their owner's full attention, especially lively breeds, who become more sensitive to a drop in attention.
