This is because seeing their armpit says a lot about your relationship to them and how you view them subconsciously. If you would ever like to know how you perceive others in the deepest annals of your subconscious then this is how you find out.
The armpit is usually derided as one of the most disgusting and filthy parts of your body. They are usually covered up by shirt sleeves and other clothing so that you don’t have to deal with looking at them and so that the people that have the armpit hairs on their body don’t need to feel so judged by others around them. This is a good thing for the most part but it also means that when they do show their armpit hairs to someone it means that they are very comfortable with them. Therefore if someone in a dream of yours is showing you their armpit hairs, or has them visible when you are spending time together, it shows that they recognize that you will not judge them and that they feel totally free around you. When you show them your own armpits in your dreams it usually means that you feel just as comfortable around them as they do around you. This is how you know who your true friends are and who you really trust in life.
If you have a dream in which your armpit hairs are growing, this recalls their inextricable link to masculinity. Maybe you are feeling particularly masculine lately or maybe something else is going on with you. If you are very young then perhaps this is a sign that you will soon be experiencing a growth spurt or the first steps of puberty. If you are a woman and you see your armpit hairs begin to grow in your dreams, this usually signifies that you might be getting more in touch with your masculinity lately.
This is not meant to be an insult; in fact it can be quite a good thing to be will in touch with your more masculine attributes. Sometimes men have dreams about having their nails painted or wearing high heeled shoes. There is nothing wrong or unusual about having a latent desire to be more masculine or more feminine or more like the opposite sex. We possess elements of both in all of our bodies and psyches and it is important to be expressive.
In fact it can even be unhealthy to not express these elements of yourself. You need to let all parts of yourself be expressed at some point even if they aren’t publicly exercised. You cannot let yourself be holed up in a box without the ability to do what you want when you want. Be more expressive and try to be more comfortable with yourself despite society’s expectations.