Sweat is not just water it is also extra salt kind of like tears it is extra salt that your body does not need so it ..
Sweat itself doesnt smell! Body odour comes from bacteria in your skin that breaks down and feeds on sweat secretions coming from your sweat glands.. 
I don't mean to gross anyone out but for the purpose of science I shall also mention that the warm and damp conditions in our armpits is also great environment for bacteria to thrive and feed hence that smell.

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