For an empire to develop, the dominant group of people must have the resources and force to extend their influence, by force if necessary, to areas beyond their borders. That being the case, the earliest true Empires would have been those of Mesopotamia, such as the Assyrians.
There is also, of course, the NEED for resources which cannot be easily accessed locally. The Roman Empire, the British Empire, even the Spanish Empire in the Americas were all dominated by the need to find additional resources, be it food supplies, or as in the case of the Spanish, gold and silver.
I think a primary reason for the formation of great empires, but also empires in general, is the acquisition and maintenance of resources. Notice Britain and Japan, both powerful island empires, relied on the resources that they obtained abroad in order for their empires to continue. Spain relied on the consistent importation of gold and silver for more than 80 years to finance their empire. Where would the US empire be without access to relatively cheap foreign oil? So I would say that control of resources are central to great empires' formation.
Wow! Interesting question.
This depends a bit on what you're calling a "great empire." For some empires (like those of the Mongols or China or even ancient Egypt), there was always competition between some settled, agricultural people and nomads. This led both sides to have to get more and more powerful until whoever won between those two could conquer other peoples as well.
For more "modern" empires like the British Empire or the Roman Empire, I would argue that technology played a major role. The British had better technology than the people that they conquered as did the Romans. This edge allowed them both to create empires.
The main reasons for the formation of great empires have been the mastering of productive cultural centers, eliminating rivals by domination, eliminating the nomads from the steppes / desserts by fortifications and superior military technology and not ultimately , the organization of intercontinental trade. Thus, the main aim of each new center of power, even if big or small, was the expansion, the concentration of smaller units in growing power centers. Such great empires have been unstable, on long term, even if the idea of building them has persisted, as the example of the Ottoman Empire. Conversely, in China and Iran, the structures of great empires have been preserved, on a more solid foundation (China) or less solid (Iran), after short interruptions (China) or long ones (Iran).