There is a well known psychological aversion to loss in humans. Studies show for instance:
A common study might show that if you are presented with a bunch of different pens, you wouldnt really have a preference for one over the other. Buf if I give you one of these unremarkable pens and then ask you to trade for another, youll hold on to this pen as if its more valuable than mine, just because its now "yours".
http://en.m.wikipedia.org/wiki/Loss_aversion
A common study might show that if you are presented with a bunch of different pens, you wouldnt really have a preference for one over the other. Buf if I give you one of these unremarkable pens and then ask you to trade for another, youll hold on to this pen as if its more valuable than mine, just because its now "yours".
http://en.m.wikipedia.org/wiki/Loss_aversion