What are the reasons of deviation of route?

Question

What are the reasons of deviation of route?

Answer 1

Weather, How much air traffic is in that certain route/ airspace, Some times atc gives the ok if the pilots want to shorten their route, Turbulence, Military activity, and if there is anything on the TFR map that may interfere with your route.

Answer 2

Well, the answer above covered it pretty well, but left off one main reason... alleged Elvis sightings.

Answer 3

Any number of reasons, weather (extremely turbulence, thunderstorms, lightning) along route of flight, slower traffic ahead, traffic cleared ahead and new route is faster, mechanic issues with navigational aids along route, airspace conflicts (like Air Force one is in the area and they want you to stay clear), congestion in an area.  On smaller aircraft it can be because they want you to stay clear of terrain or congested areas, even military activity.

It is almost always to promote safety of flight and possibly a faster route.