How come people say dogs can't have chocolate?

Chocolate has a certain stimulant in it called theobromine, which is naturally found in cocoa plants. This stimulant basically has the same effect as a sugar rush, and this is why it is harmful for dogs. If a dog were to have too much, it's system will try to get rid of it, which would result in vomiting/diarrhea, and will cause discomfort to the dog. These are the sypmtoms that would appear before the actual "sugar rush" effect would take place: the dog's heart rate would increase, and if not taken care of, the dog's heart may actually give out in the end, ecspecially if they have had medical problems in the past, or are older. Of course, if your dog has developed his/her immune system towards it, they won't be as affected, or affected at all, ecspecially if you are only giving him/her a little. Even different brands of chocolate have varying levels of theobromine in it, so technically some is less harmful then others. It would be wise to be cautious about it, but there is no need to be paranoid.