This was especially true before the industrial revolution.  Since there was a very limited range for development of resources, monarchs decided it was easier to take land and resources instead of developing pre-existing ones.  Greed most definitely played a part, but it eventually became a pretty vicious cycle (Spend money on armies, not industry, go to war to gain resources and constantly have do defend them as well).
 It ended with the World Wars because these newer governments realized that it was much more effective to develop resources and industrialize than to constantly be at war.  In my opinion, the World Wars corrected all of the land that has over time been conquered by various nations, and gave it back to the lands "Rightful Owners", I also believe that war was much more accepted in those times than it is now, especially given the rise of individualism over statism. 
 The answer to this question is that it was a much different time back then, and there is no single reason or answer to the question, but I hope his helped!