"’I thought chocolates weren’t very good for dogs,’"

Chocolate is bad for dogs because it contains theobromine, which dogs can not metabolise as well as humans can. It can cause digestive problems and possibly lead to heart failure, particularly if consumed in large enough quantities.

Sources

http://en.wikipedia.org/wiki/Theobromine_poisoning

Why is chocolate bad for dogs http://www.associatedcontent.com/article/32454/why_is_chocolate_bad_for_dogs.html