Humans believe that they are not constantly smelling each other. Despite the best efforts, it's a known fact that everyone has a pleasant and unpalatable scent, and if it is like other land mammals, one's unique aroma may mean something to the other fellow humans.
Some of these are self-explanatory, such as the stench of someone who hasn't bathed in a month or the distinct voice of a baby claiming they didn't fill their diaper.