Finally, I concluded that the only link that made any sense was to associate it with lost youth. Close your eyes while listening to it and imagine a montage of Kennedy playing with his children, walking with his brother Jack, sitting on the sand dunes looking out over the ocean - then, and only then, could you really begin to figure out what Bernstein might have been getting at. I have no idea if he had anything remotely like this in mind, but as an explanation it worked for me.
Over the years I've gained a great appreciation for Mahler, and this piece in particular. And it is in that spirit of lost youth, of the heartbreak of pondering a future that never was and a past that never will be again, that we listen to it today, and think of those souls who never had any idea of what hit them, nine years ago, and of those who did, and were powerless to do anything about it. The question, nine years later, is this: are we? ◙