Breed

• The tear ducts of some large-eyed dogs, Chihuahuas in particular, weep frequently to keep the surface of the eye moist and free of debris.
  
  

Food Allergies

• Weepy eyes can result from allergies to commercial dog foods. HolisticPetInfo suggests reading labels and avoiding "BHT, BHA or ethoxyquin and food coloring."
  
  

Irritants

• When eyelashes grow inward toward the eye or when foreign matter (like a speck of dust) gets trapped beneath the eyelid, a dog's eye weeps in an attempt to dislodge the irritant.
  
  

Blocked Ducts

• Dogs' tear ducts drain into their nasal cavities in part to keep the nasal passages and snout moist. Dogs with narrow or partially-blocked pathways may experience excessive external tearing, or weepy eyes.
  
  

"Rust"

• VetInfo says the "rust" stains on fur from weepy eyes are because of iron-carrying molecules being broken down by bacteria. Though tetracycline can help, it's best to check for problems like glaucoma or uveitis.