the sciency type answer is that we have a lot of emotional baggage attached to smells which is some of the reason (not all) that we find them nice or nasty. Dogs don't have the emotional baggage, their classification is different...edible/not, mateable/not, of my pack/not and so on. Dogs do find some smells unpleasant but they base it soley on the smell, not what the smell conveys unless they have learned a reaction, eg to the disinfectant used in the vet's waiting room. Even then I would guess that they don't actually dislike the smell but dislike what the smell means to them.
This is also part of the reason why dear little kiddies are happy to fingerpaint with the contents of a nappy....they don't yet have the "yuck" connotation installed