Without doubt the cause of psoriasis down to the individual's own immune system malfunctioning - on other words it's a an auto-immunity disease.
In this way, psoriasis is related to other immunity diseases such as allergies and asthma. In plain English, the actual cause of psoriasis is the body's immune system attacking its own tissue. Not recognizing the make up of its own tissue, the immune system treats the body's own cells like harmful invaders and attakcs them.
The immune system therefore causes the epidermis cells to form too quickly and they start piling up on the skin's surface. At best, this causes flaking of the skin and scaly red patches, at worst it leads to cracking splitting, bleeding, soreness and aching in the joints. Also, while not specifically a cause of Psoriasis, there is some evidence to suggest that stress, or emotional duress often brought on by the disease, can result in outbreaks and further an already bad case.
As is the case with many skin ailments, stress can not bring on an attack of the illness in an individual not already suffering from the disease, but it can bring a case out of remission, or advance an existing outbreak. It's very important that sufferers take steps to regulate not only their skin's condition, but also their emotional and psychological wellbeing.