The most common question I get when I see a patient with psoriasis is "what caused it?" or "how did I get this?" The answer is complicated, but I've distilled it into the simple comment that "there is a genetic predisposition and usually a trigger factor that activates the disease". I think that's about as simplified an explanation as you can get.
A group of researchers in China compared 178 patients with psoriasis to 178 control patients with normal skin. What they found was that having the genetic HLA markers called HLA-Cw6 and HLA-B27 increased the risk of developing psoriasis by 2 to 10-fold. they also found that the risk of psoriasis was increased by environmental factors such as manual work, marriage, stressful life events, obesity, family history of psoriasis and smoking. So although you can't choose your parents, you can try to live a less stressful life, maintain normal body weight and don't start, or quit smoking. I have no comment on the part about marriage!