Entropion is a common congenital defect of the eyelids 
It is usually due to an inherited faulty eyelid conformation that manifests in young adult dogs.
 
Sometimes entropion may be caused by dust or debris collecting in the eye
Some cases are due to the fact the eyelashes are  growing in the wrong direction
In  some animals it is due to an eye injury or problem.

Entropion is the term given to the overall condition when the eyelid inrolls , this brings the eye lashes into direct contact with the cornea.
Thus this contact causes and creates an irritation and the animal blinks constantly 
when the animal blinks constantly it is adding to the problem by scraping the eyelashes across a more extensive area of the eye

It is a painfull and irritating condition and needs to be sorted out quickly and efficiently.
Entropion requires vetinary intervention.
 It is usual for surgery to be reccommended to give the animal a better quality of life