Power, Religion, War, hunger/famine, Control, Territory claims. These are a few of the reasons. Either way all these factors caused a lot of termoil and of course war. Families with power rose, sacrifices were made, and people migrated to escape slavery, hunger and oppression and perhaps weather. You can pretty much weight out all the pros and cons, especially since many did not make it back at times but it was a way of survival. Without change we'd all die.
