No one really knows for certain what causes psoriasis. But there are two factors that may contribute to the development of the condition-
1. Genetic predisposition
2. Triggers that make psoriasis appear.

Genetic Predisposition:

Psoriasis often, but not always, runs in families.
It seems that what happens is that you inherit a tendency to psoriasis. But the condition then develops only if you are exposed to specific triggers in the environment, such as viral illnesses, certain drugs or stress.

 Triggers that cause psoriasis:

If you have genetic predisposition to develop psoriasis, certain triggers may activate the condition. These triggers can be –

 ACCORDING TO AYURVEDA:

1. VIRUDH AAHAR( FOOD COMBINATIONS I.E.,Using of opposite quality of foods at the same time)

Using Yoghurt with Milk, or Fish with Milk, or ACHAR (Pickle) with Milk or Honey with Ghee (Butter). These are opposite foods and if taken together, they produce a lot of toxins in our body.
Taking fish preparations and cheese preparations along with alcohol, this is a deadly combination and can easily lead to skin diseases, because fish with milk products like cheese are contraindicated.

Taking heavy meals in short durations without getting earlier meals properly digested also produces toxins which gets released in the blood and in due course cause psoriasis.

Having sex within one hour of taking meals can cause psoriasis by delaying the digestion process of meals thereby producing toxins, because one hour is minimum required time for our body to secrete the juices to digest the meal. If our attention is diverted towards sex then these juices are produced in less quantity in which case the undigested food will get converted into toxins.

Stopping the natural Vegas (normal phenomenon of our body) can cause psoriasis. Passing Stool, Urine, Flatus and Semen are natural Vegas, which should not be stopped. If we are having constipation then part of the toxins present in stool gets re-absorbed in the form of toxins into our blood system and they accumulate in the body to produce skin diseases including Psoriasis.

According to Ayurveda-there are three pillars which support our body system.
1. AAHAR(FOOD)
2. NIDRA(SLEEP)
3. BRAMCHARAYA(SEX)

A person who is takes a healthy balanced diet and avoids virudh aahar and healthy lifestyle as discussed above, takes a good and sufficient night sleep and indulges in good sex practices is less likely to be effected by Psoriasis, even though he might be having a family history of Psoriasis.

 ACCORDING TO CONVENTIONAL OF MEDICINE-

HORMONAL FACTORS-
   There is a peak onset of psoriasis at the menarche ( when the periods first start) and the menopause( when the periods stop). Psoriasis tends to increase with pregnancy and worsen after delivery. However, this is by no means invariable and there are no clear cut hormonal triggers.

‘TRAUMA’ TO THE SKIN-
If the skin is injured, psoriasis may develop. This is known as the Köbner reaction.

SUN EXPOSURE-
   Ultraviolet light helps the vast majority of people with psoriasis. However, a few people(less than five percent) notice that even small amounts of sunlight- at levels that do not cause sunburn- aggravate their condition. Some people will have psoriasis only on areas of their skin that are often exposed to sun, such as their face, hands and forearms. If a person is sunburnt, there is a risk that psoriasis will flare up because of above mentioned Köbner reaction.

DRUGS-
   Some drugs worsen already existing psoriasis. This applies particularly to Lithium, which is commonly used in management of depression. If you have been taking cortico-steroid drugs by mouth for other conditions, you may find that your psoriasis flares up when the cortico-steroids are withdrawn. The same reaction may occur if you have been using potent topical steroid creams and ointments on your skin. If you suddenly stop using your topical steroid creams, a nasty and often pustular flare-up of psoriasis can result.

PSYCHOLOGICAL FACTORS

It is always difficult to prove a relationship of the mind to psoriasis. Undoubtedly, in some people, psychological factors are important and they notice that when they are under stress their psoriasis worsens. On the other hand, it is stressful to have an unpleasant skin disease, so it is hard to sort out the cause and effect. In individual patients, stress can be a clear aggravating factor, but, for the majority, it is not a strong trigger of psoriasis.