Why do my armpits stink and what causes the smell?

Your armpits stink because bacteria love dark, wet places.

You sweat there, creating moistness, and because your arm isn’t always raised ¬ówhich would allow constant airflow, ¬óbacteria begin to multiply.

You don’t really stink, per se, it’s the excrement of the bacteria that stinks.

The general advice here would be to have a shower as soon as you can.