Why Do Empires Form?

Empires can form to unite a group of people.  This has happened in India with Chandra Gupta, and in China with Qin Shihuangdi.  On the other hand, empires can arise from people just wanting more power.  For example, in Rome, more and more people wanted power over the lower class.  Higher class citizens kept gaining more and more power and land until it was a full blown empire.  So, to answer the original question, empires form because of want for power.  It can be for good, to unite people, or for worse, to gain more control.  Either way, that is how the great empires of the world have come about.