The pilots wanted a change to scenery. They were tired of seeing polar bears.

The real answer is the jet stream, which goes in an eastward direction. It can increase the ground speed of the airplane by 100 mph. Going west, it's in the pilot's best interest to avoid it. Hence, a polar route is used going west.

Example, I flew IAD-NRT, and flight took the shortest path distance-wise. However, on the way back, the flight flew over SEA, which added around 300 miles to the trip. But the jet stream was very fast. Here's a visualization.

http://gc.kls2.com/cgi-bin/gc?PATH=i...OR=&MAP-STYLE=