I wish I could say exactly what it is that I like so much about this movie.
Maybe it's the comedy of it. Humor is deliberately absent from most Western films, and it's twice as fun when it's there for that reason alone. Butch creates most of the fun, not because he's a comedian, but because he's an extrovert who's always on top of his game, or at least is always trying to be. Even as we're looking straight toward the tragic ending of the main characters, we're given one last exchange of bright-eyed dialogue between two men who know each other inside and out.
Maybe it's the "buddy film" aspect, where two guys can be renowned criminals and still have complete trust in each other. They complement each other well, Butch being the cleverest bank robber around and Sundance being the best shooter. No matter how many members of their gang decide to break rank, or how hard they're being chased, or how much time they're spending with the same woman, they stick by each other as if it were unthinkable to do otherwise.
Maybe it's the fact that it's such a great guy movie. Robert Redford and Paul Newman were in their prime when this movie was made, and they play their parts well. This isn't a macho Western, it's about two guys who try to use their brains and skill to get around the sort of gunplay and fistfighting that Clint Eastwood would have just waded through.
But I think what it really is is a combination of all of those, plus the whole tragedy of their lifestyle. Like the Sandman and Hamlet, Butch and Sundance realize and acknowledge that they are bound by who they are and what they do. They're bank robbers, very good ones, and they enjoy what they do so much that it's become a part of who they are. They try to give it up, but they can't. And after they're done trying, and they know without question that death awaits them at the impending end of their career, and accept it.
It's a crazy, romantic fatalism that people throughout history have understood and admired. Their lifestyle may not be moral, but nevertheless, they are willing and ready to die for being who they are. I think that deep down, when our end comes, we all want to be able to say the same.
I make no apology for giving away the ending to this film. It's a true classic, and if you didn't already know it, you should before seeing it. Like any other tragedy, the fall is what makes the ride worth watching.