Why does a dogs urine smell like ammonia?

Concentrated urine often carries a strong ammonia-like smell to it, particularly to sensitive individuals. This is from the urea in the urine, which is the waste product of protein metabolism - it's normal for this to be present, and generally concentrated urine means the dog's kidneys are working well.

However, if your dog's urine suddenly starts to smell very strongly of ammonia, I would keep a very close eye on him to make sure he isn't dehydrated or becoming filled with edema fluid. Dehydration is usually associated with decreased energy, decreased water consumption and a general attitude of "I don't feel good". If your dog is accumulating edema fluid, you'll notice the lower legs and belly starting to become swollen; you may be able to push a finger into the swelling and make a small dent that slowly fills back in. Both of these are signs of more serious medical issues that would need to be examined by a veterinarian.