When your body sweats it mixes with the dirt collected on your skin throughout the day to cause itchy armpits.
When exercising you should wear loose clothing to give your armpits the chance to breathe and prevent rubbing against your skin.
Bacteria is an irritant and is one of the biggest causes of underarm itchiness, so try to spend a little more time on hygiene in your armpits. When your skin is wet it's much easier for bacteria to collect and spread, and once this happens your skin is susceptible to infections that cause things like itchy armpits.
Shaving without a moisturising cream or gel, with a dull razor, or even too often, can all cause itchy armpits.
Laundry detergents contain different kinds of soaps designed to remove hard stains and dirt from clothing, so they often contain strong chemicals that react with your skin causing those itchy armpits.
Some soaps contain strong and harsh chemicals that cause your skin to become dry and angry, especially in sensitive areas like your armpits. Avoid body washes and soaps containing strong perfumes and colours which can provoke skin irritation leading to itchy armpits.
For some people it's a little more complicated and skin is prone to irritation from not only harsh chemicals but a number of different things, including deodorants. To avoid itchy armpits when you have sensitive skin you should use perfume-free deodorant and preferably one without aluminium as this clogs your pores to prevent sweat, and instead you should let your skin breathe to reduce irritation.