Dogs are known for eating things when they are not supposed to. This is especially true of puppies. Also, dogs have an excellent sense of smell, making it fairly easy to find any secret hiding spots for the chocolate. This can be a dangerous combination when there is chocolate around the house.
Chocolate is derived from the roasted seeds of Theobroma cacao, which contains certain properties that can be toxic to animals: caffeine and theobromine. If ingested, these two ingredients can also lead to various medical complications and may even prove fatal for your dog.