What makes Chocolate so toxic for Dogs?

Hershey’s Chocolate and dogs
How harmful are Hershey’s products for dogs?

From Kirk and Bistner's Handbook of Veterinary Procedures and Emergency Treatment 6th edition.
The active ingredient in chocolate is theobromine:

The half life in the dog is 17.5 hours

The Toxic dose in the dog is 100-150 mg for every 2 lbs of bodyweight in your dog..
For a 50 pound dog, a toxic dose would be roughly 2.2 grams (2200 mg) of pure chocolate.
The concentration of theobromine varies with the formulation of the chocolate, therefore:

Milk chocolate has 44mg/oz (154mg/100gm):
toxic dose for 50 lb dog - 50 oz of milk chocolate.

9 Hershey’s Kisses contain 91 mg of theobromine

Semisweet chocolate has 150 mg/oz (528mg/100gm):
toxic dose for 50 lb dog - 15 oz of semisweet chocolate

Baking chocolate 390mg/oz (1365 mg/100gm):
toxic dose for 50 lb dog - 5 oz of baking chocolate

Thus a dog eating one oz of baking chocolate would have to eat almost 3 oz of semisweet or 10 oz of milk chocolate to get the same dose of theobromine.

The theobromine in candies consisting of chocolate that is coated over some other substance - as in filled candies and chocolate coated dried fruits, etc. will be more dilute than that in pure chocolate bars and solid chocolate candies.

Obviously the chocolate in milk chocolate is quite dilute and this is why many dogs can eat a piece here and there and seem not to show toxic effects, how many dogs would get ahold of 50 oz at a time? This is not true of the more concentrated forms however. Dr Sue Bank's experience was that she had two dogs, a 95 pound one and a 60 pound one. They got ahold of 2 one pound bags of Nestle's semi-sweet chocolate pieces (a bag each). The 95 pound dog survived but the 60 pound dog ingested a toxic dose.

The problem with feeding a dog milk chocolate as a treat is that it develops a liking for chocolate and since dogs do not seem to be as sensitive to bitter tastes as humans - it may then eat the more concentrated, and thus quite toxic, baker's chocolate if it gets a chance or it will consume a toxic amount of milk or semi-sweet chocolate if it can get into a improperly stored supply.

Treatment which is best administered by someone with medical training follows the same strategy as treatment for caffeine overdose:

Support Respiration
Support cardiovascular function, control arrhythmia's, control electrolytes and acid-base balance.
Control CNS excitation.
Gastric lavage
Activated charcoal

Administration of an activated charcoal slurry is a major component of the treatment and needs to be administered by a veterinarian - it is not a home treatment.