Keeping up with the Joneses

What is it about people that make them feel like it is necessary to one-up people?  I have been thinking about this a lot, and I can't comprehend this mindset, I guess because I don't have it.  If one of my friends were to purchase a mansion on a hill, I would truly be happy for them.  Truth be told, I don't want a mansion on a hill; somebody has to clean that monster, and I just don't wanna.  But if I did, I still wouldn't feel the need to out-do them just to say I had something.  It's just silly to me!  Didn't you work hard for whatever you have now???  Can you never just appreciate where you are and be proud of yourself for getting there?  At some point, aren't you worried that someone is going to call you out on being a copycat and tell you they don't want to play with you on the playground??