My daughter's dog a young puppy stole a square of Galaxy chocolate from the table, my daughter phoned her vet and told her what he had done, the vet said one bit should be alright but keep an eye on him.
Question, how come chocolate is so bad for dogs but okay for us humans?

When I had a dog I was told by the vet that chocolate was toxic for dogs. It contains theobromine a caffeine type chemical which makes dogs ill. I was also told not to let my dog eat raisins or grapes. He once stole and ate a chocolate from a box on the coffee table on Christmas day but he was okay.
Raisins and grapes can damage a dog’s kidneys. I never knew these things until I owned a dog.

Darker and more expensive chocolate usually have more theobromine and is more likely to be harmful to dogs.
It’s the type as well as the amount of chocolate a dog eats that will determine the outcome.

Pats is right. The darker chocolate that we consider more healthy for us humans is much more toxic for dogs than milk chocolate. It's always best to consult a vet if your dog eats chocolate.

If it is bad for dogs would it affect us?
Saying this as I am eating a chocolate orange.

our digestive systems can process theobromine much more quickly than dogs do - so the toxins build up in their bodies.

Same but different - birds can eat holly berries but people and cats/dogs can't.

Another of Nature's many wonders.

Years ago I had an elderly toy poodle ,unbeknown to me he ate an entire packet of chocolate covered nuts , I found the empty packet in his bed.
He suffered no ill effects .

Years ago my daughter’s labrador ate the contents of a selection box from under the Christmas tree one Christmas with no ill effects.

We came home once to find our Jack Russell had eaten all the mini milky ways, mars bars etc in the selection bowl. The wrappers were all over the floor. She suffered no ill effects either. I suppose it depends on the quantity. My friend has been giving her dogs grapes for a treat until I told her they were dangerous for dogs. The dogs have not been ill. Again,, quantity, I think.

Dogs shouldn’t eat grapes, raisins, currants or sultanas.
If eaten, these fruits can be highly toxic to dogs and can kill them.
Not all dogs are affected in the same way, some may become more unwell than others ie developing kidney failure.