If I recall correctly, Dorothy Rowe (psychologist) described "mental illnesses" as "desperate defences" against our "greatest fear". According to Rowe (if I recall correctly) Extraverts' (as defined by her) greatest fear is "total abandonment" and Introverts' (as defined by her) greatest fear is "complete chaos"...

(I haven't read Rowe for many years although I always liked her, so forgive any mistakes or inaccuracies )

I have certainly found her more helpful than any medical "help" for Depression or other mh dxs? in the UK (but, that's just me...)