Reasons and motivations (and actions) do not need to be disclosed in order to make a compelling film. I can only offer a bit of advice, and that is to listen. I'll never tell just what to look out for, but after my first viewing, I wished that I had listened more carefully to the beautiful exchanges in dialogue. Much of it taken directly from McCarthy’s metaphorical words, it is essential to contribute your senses to the characters (namely in the third act) in order to walk away with some form of notable coherence.
Startlingly violent, smart, witty, frequently humorous and wonderfully paced, No Country for Old Men is the best film the Coen’s have ever done and quite possibly, the best film of the year.