I've heard chocolate is really dangerous for dogs to eat.  Why is this?

A toxic dessert.

Chocolate contains methylxanthine alkaloids in the form of theobromine and caffeine, that cause constriction of arteries, increased heart rate, and central nervous system stimulation. These effects can lead to vomiting, diarrhea, restlessness, and increased urinations. More advanced symptoms of toxicity include excitability, increased respirations and heartbeat, stiffness, seizures and exaggerated reflexes.

Certain types of chocolate contain higher amounts of methylxanthines, baking chocolate containing the highest and white chocolate containing the least. Dogs freely ingest toxic amounts of chocolate if it is left accessible. A potentially lethal dose in a 16 pound. dog is only one pound of milk chocolate. People stop eating chocolate before ingesting toxic levels.

Cardiac failure, seizures, coma, and death can result if the chocolate ingestion is not found within four to six hours and treated appropriately. The length of action of the methylxanthines is usually 12 to 36 hours. In dogs that chocolate ingestion is detected early, the prognosis is good.

If your dog has gotten into chocolate, you should note the type, estimate the amount eaten, and then call your veterinarian for recommendations. You will probably need to take your dog in for examination and evaluation of heart rate. If your veterinarian is not available, you should seek emergency care. We recommend that you keep your chocolate in an unreachable location; an ounce of prevention is better than an ounce of ingestion.