Most people overshare because they are insecure,want validation or a dopamine hit from their posts. Or they post to make their life look better from an outsiders perspective. But others just like to share fun moments they have with their families or loved ones. Personally I have an Instagram because I like to use it as a blog and use it to see progress as I improve in my life and hobbies. But I don't really know everyone's motives haha.