Why Can't My Dog Open His Left Eye?

This can be serious if he experienced a traumatic event that resulted in the rupture of his globe (eyeball).  It would be best to have him examined by a veterinarian to thoroughly examine the eye and surrounding structures.  Medication can then be prescribed and may include topical eye medication, pain medication, and anti-inflammatories.  His oral cavity should also be examined.

Could have something stuck in it. Around here a lotta dogs get cheat grass/fox tails stuck in their eyes. If there is something in it, it will scratch the eye and cause ulcers and could lead to worse things. You can try flushing the eye and gently use a q-tip to check for something. The dog could just have an eye cold and just needs it kept clean with a warm wash cloth. If you can't find it, you should take the dog to your vet.

My dog has the same problem. His eyes kind of looked fogged, and glazed. He's whimpering a lot, too. I never saw him get scratched though...