As movie-goers, we've been conditioned to believe that films can be broken down into a beginning, a middle and an end; we sit down to a film and instinctively feel the natural rhythms of the story as it unfolds, anticipating the end credits to arrive at a point when events on-screen appear to be winding down.
There are lots of movies which don't quite know when to end, of course - they go on and on, piling unnecessary scene upon unnecessary scene, desperately searching for the right place at which to finish the story. In the process of doing so, though, the overall narrative ends up feeling flabby.
Even worse than that, though, is when a movie ends in a totally abrupt manner at a seemingly crucial moment in the story; right when, having glanced at your watch, you presumably just told yourself there were at least 30 minutes to go.
Nope: it's at this point that the screen cuts to black and you're ushered out of the theatre in a state of total confusion, wondering what happened to the final reel or why the heck the filmmakers were in such a rush to wrap things up...