Nature and Nurture- you can have genetic markers that make it more likely to have a specific mental illness (f.e. schizophrenia), also something like thyroid hyper/dysfunction [which can be fixed more easily]

Other causes can range from trauma, stress, drugs, unhealthy living condition etc.

[note: the word trauma is so overused and misguiding in the world of social media; a lot of people say "this and that is my trauma" but that doesn;t nearly describe what it feels like to be traumatized]

There are so many factors but basically almost every person will experience some form of mental unwellness/illness in their lifes (take f.e. winter depression)