I did therapy when I was younger. My therapist was an introvert and considered me one too. Her belief was that introverts were not so easily accepted into the world and that I would best learn to enjoy it.
I think my grand daughter is an introvert. When she was tiny she didn't even like being hugged much. Her Mum was a bit upset but just went with it. Now she is all love. She is beautiful and genuine. She is herself...but although she is only 20months, she takes her time, she looks around, she puzzles things out and when it feels right for her she has a ball.
She is being given the space to be herself. That I think is sometimes what the introvert does not get and that I think is what causes problems.
Depression on the other hand is I think a different need to be alone and can happen to both introvert and extravert's.