Bacterial or infections like abscesses or styes cause eyelid and conjunctiva or swelling, resulting in one-eye squinting as a response.

Eye trauma can make a cat squint in one eye due to pain. Scratches and abrasions, or lacerations are common causes for eye squinting.

Allergies can cause swollen eyelids and conjunctiva, leading to one-eyed squinting due to insect bites or systemic reactions.

Foreign objects in the eye cause irritation and squinting  or discomfort in the affected eye due to eyelid and conjunctiva irritation.

Entropion in cats causes inward-turned eyelids, leading to eyelash rubbing against the cornea or discomfort, and squinting in one eye.