Despite the most adamant testament to the contrary, we all smell (though some more pungent than others). Body odor is simply a natural part of being a mammal (thank goodness for soaps and deodorants!).
Curiously enough, though sweat is usually blamed for body odor, sweat itself is odorless. The cause of unpleasant odors result from the skin bacteria metabolizing the sweat into compounds that have scents. Thus how we smell depends on many factors: how much sweat is produced, the types of bacteria living on our skin, the foods we eat, and the medications we take.
But despite the most rigorous hygiene routine, sometimes bad body odor can't be helped. In the most extreme case, a genetic condition known as trimethylaminuria, or fish odor syndrome, causes people to give off an offensive smell of rotting fish in their urine, sweat, and breath. Though dietary restrictions and use of acid soaps and lotions help the odor, there are no real remedies that can completely mask the fish smell.