Cherry eye in dogs is caused by a prolapse of the gland of the third eyelid, also known as the nictitating membrane. This gland is normally anchored to the lower inner rim of the eye by fibrous tissue, but in certain dog breeds, this attachment is weak or defective, allowing the gland to prolapse or pop out of place

. The exact cause is not fully understood, but it is believed to be hereditary and related to genetic weakness in the connective tissues that hold the gland in place. The condition is more common in young dogs under two years old and in breeds with certain facial structures, especially brachycephalic (short- nosed) breeds such as Bulldogs, Beagles, Boston Terriers, Shih Tzus, Lhasa Apsos, and Pugs

. In some cases, sudden excitement, shock, or trauma to the eye can trigger the prolapse, but most often it is due to the inherent weakness in the ligamentous attachments

. In summary, cherry eye is caused by a congenital or hereditary weakness in the connective tissue that anchors the third eyelid gland, leading to its prolapse and visible red mass in the inner corner of the dog's eye