Great Expectations . . .
 

The good old "Grass is greener on the other side" situation?
There are lots of possible reasons, the first few to spring to mind are:
1) The media and society in general surround us with the view that we material possessions can lead to happiness. When we fail to achieve this we assume it is due to a lack of the correct type of possession.
2) People set their expectations of things to high and so once something is possessed and fails to live up to the expectations they start to look for a new item to fill the gap (often without stopping to think whether the thing does what they need or not)
3) People are just fundamentally screwed up and are psychologically incapable of settling with what they have. Rather ironically this is good for mankind as a whole since it drives innovation but a bummer for the individuals involved since it means that most people are unhappy most of the time.
 

...Spoken like a true engineer.

;P