Matt Yglesias puts it this way, analyzing Kennedy's most famous speech:
Its closing line is, I think, crucially important: “For all those whose cares have been our concern, the work goes on, the cause endures, the hope still lives, and the dream shall never die.”
I’m never able to express myself nearly that well, but what I take Kennedy to be doing here is trying to offer an alternative to the boom-bust mentality that often overtakes American progressives. There’s a tendency to get extremely wound up with optimism about the imminent dawn of sudden and radical change for the better, and then intensely bitter, cynical, and depressed when that fails to materialize. The reality, however, is that change is hard. That’s not an excuse for the people who stand in its way, it’s the reality. But if you respond to the difficulty of making things better by giving up or getting frustrated, then it only gets harder.
Building a better country and a world is work—hard work—and it’s work that goes on. And on. And on.