Why can't dogs eat chocolate?

There are chemicals in the chocolate that are toxic, they can cause seizures and death in some cases.

Theobromine, a caffeine-like molecule that increases blood flow to the brain, is so hard for dogs to process that they can easily eat enough to be poisonous. Few dogs eat fatal amounts of chocolate, but smaller doses can still cause seizures and other symptoms.

Chocolate toxicity for dogs

Does makes the poison. Our vet has a circular slide rule to do the calculation – amount of chocolate eaten vs weight of the dog.