This can happen for several reasons, including your genetics, irritation from products you're using, or an underlying condition – which should be ruled out by a doctor first! 

Want to do something about it?

If you're unhappy with the cosmetic appearance of your underarms and have ruled out any possible conditions with your doctor, look to formulas featuring Niacinamide, Retinol, Kojic Acid and Hydroquinone which may help to lessen the appearance of pigmentation.