Why do my eyes constantly water when i have a cold or allergy attack?

Nerve interaction: There are connections between the nasal sensory nerves and the lacrimal glands that produce tears. Therefore anything that irritates or inflames the nose induces tear production. That explains the reduced tearing when nasal steroids are used to treat allergy. The steroids reduce the allergy response and nasal inflammation with less tearing. Oral and topical antihistamines will reduce itching.

Blocked exit : The exit/drainage routes for your tears may be swollen closed by allergic conjunctivitis. There may be reflex tearing that over loads your tear drainage system even if the exit routes are not swollen closed. If your nose is also running, the exit routes are working and it is the excess reflex tearing.