Which ones are the most common causes of mental illnesses and why is that?

Mental health illnesses can occur due to: 

1. Environmental variables: Substance misuse, poverty, abuse, neglect, trauma, and other environmental factors can all have an impact on the development of mental illness.
2. Brain chemistry: Chemical imbalances in the brain can result in the onset of some mental diseases, including bipolar disorder, anxiety, and depression.
    
3. Developmental factors: Early childhood trauma, abuse, and neglect might raise a child's chance of adult mental illness.
4. The origins of mental diseases can vary, and in many situations, a number of different factors are likely to work in concert to generate a particular problem.
5. Genetics: Certain mental illnesses can be passed down through families through genes, making some people more prone to acquiring a specific disorder.
6. Physical disease or injury: Mental diseases can arise as a result of brain injuries, chronic physical illnesses, and other medical issues.

It's important to note that mental illness can arise from a combination of these factors, and not all individuals who experience these factors will develop a mental illness. The exact cause of mental illness can be difficult to pinpoint, but understanding the possible causes can help with prevention and treatment.