One of the topics discussed on the backpacking trip was how much the boys used to whine and complain while we did this hike when they were little. This was especially galling, because The Dad carried most of the stuff in those days. Ben was especially noted for grousing in those days.
The boys had an excellent reply: Ben complained about everything, and was, paradoxically, enjoying himself complaining. When I could just leave him alone, without trying to cheer him up, encourage him, or (eventually) complaining back at him, he would do fine. He plodded along, but eventually made it.
Some kids just complain, and you really shouldn't feel obligated to intervene about it. When I left him alone to grouse, he was in his element.