Chocolate is lethal to dogs because it contains a substance called theobromine, which dogs are unable to metabolize effectively. Theobromine can cause various symptoms in dogs, including vomiting, diarrhea, rapid breathing, increased heart rate, tremors, seizures, and even death in severe cases. The severity of the symptoms depends on the type and amount of chocolate ingested, as well as the size and sensitivity of the dog. Therefore, it is important for dog owners to keep chocolate out of their pet's reach to prevent any potential harm.