It had taken WALL-E approximately seven hundred years (give or take a decade) to find all the garbage his claws could carry, compact them down into cubes, arrange said cubes into a proper order, and eventually build them up into mountainous towers whose heights he regularly visited.
It took the humans less than a day to tear them all down.
It was hard to bear. They were not delicate about it; why should they have been? These structures were special only to him. To them, they were just more scrap to be melted down for supplies, space to be freed up for new developments. WALL-E understood that, and so made no objections when the idea was brought up. He would not be so selfish as to withhold something helpful from these people just so his work could be preserved. So the robot watched years and years of hard labor fall apart in silence. It should not have felt this bad, he knew; he still had so much to cherish: his home, his collections, EVE...but these towers were landmarks...memories. They were the reminders of where he'd been, what he'd done...there was the spot he'd first met Hal, and that was the tower he'd made on the day he'd first discovered music, and ooh that was the one he'd turned into a makeshift slide...gone. Seven hundred years...gone.
The trudge back home felt particularly long that day.