The cause of psoriasis is still unknown but some factors are considered as causative factors like:-
Family history :- Psoriasis is a disease which passes on. If your one parent has psoriasis you are at a risk of having it but if your both parents have psoriasis you are at a higher risk of having it.
Stress :- a strong mental or emotional pressure can cause it.
Smoking and Alcohol :- these two factors are known to cause Psoriasis and also disrupt the treatment of an existing as they make medications less effective.
Injury:- injuries are seen to trigger Psoriasis.