The idea of your breath as a bad smell isn ' t built into their brain because they ' ve had no reason to evolve that way . Humans on the other hand have , due to various reasons . Maybe it indicated bad heath or disease , and so we ' ve evolved to naturally react and get away from the source . Smell is subjective . The dung beetle for instance is attracted to what would be a nasty smell for us . It might find other things repulsive , and yet others completely neutral , depending on what ' s good or bad for it . I don ' t particularly like the smell of rubber , but I ' m not repulsed by it either . It falls more into the neutral category for me , and I think most people are probably the same . Why ? Humans never evolved around rubber . It ' s a new phenomenon . Roses on the other hand , or rotten fish ? Yeah we ' ve been around those for a long time and our brain knows what happens if you interact with one as opposed to the other , and so it has come to react in certain ways when it encounters them . There is otherwise nothing else that would somehow make the smell of rotten fish bad . It ' s just that the brain is labeling it that way and we believe it and react .