So far I think it's a consensus that there are at least three reasons for telling someone to "get a life". 1) you care for them and you see them not taking care of themselves or 2) you think that you know exactly how they should be living their lives and that's your way because you're so much smarter then they are and want to play "God" or 3) you're just being spiteful and you know that everyone has insecurities about who and what they are - especially when they are extremely passionate about something.
