Why leave home, friends, family, the familiar environment and lifestyle that brought you up, that your education prepared you for, and take off to live in an alien country and culture?
I like to think that in my case (American), my education has prepared me to see the world as a place where there is a diversity of cultures and lifestyles, so why not travel/live elsewhere if one chooses?

What keeps me in Japan? (been here 12 years)
A job and family and a general liking of the place.