What do you, personally, think mental illness is?

I’ve been thinking about how mental illness is defined, and have been wondering what the philosophical underpinnings of the idea of mental illness are. It made me wonder what kinds of different ideas you all have about mental illness. How would you defend the idea that mental illness exists? How would you justify the idea?

Feel free to throw around blatant prejudice, but I hope you can also justify your ideas about mental illness in a logical or even scientific way, as well.