To me, the underlying theme is time and change, there are few characters whose lives remain static. There is an cleverly woven texture of childhood, youth, adulthood and old age; change and death are there at all times, dreams are created, fulfilled and destroyed.
Other threads include a typical road movie, with the actual and symbolic journeys in convoy. There is social commentary, characters drawn from all tiers of society. A narrative voice cuts in from time to time, usually to embroider the history of the characters or their surroundings, an all-knowing inner voice that gives the viewer a tidbit of information that isn't known to all, or any, of the characters.
This was one of the best films I've seen this year.
