On what principle is it that, when we see nothing but improvement behind us, we are to expect nothing but deterioration before us?

A wonderful passage from Southey's Colloquies on SOciety by Thomas Babington Macaulay quoted by Timothy Taylor in Macaulay on Economic Progress, One Hundred Years Before Keynes

Hence it is that, though in every age everybody knows that up to his own time progressive improvement has been taking place, nobody seems to reckon on any improvement during the next generation. We cannot absolutely prove that those are in error who tell us that society has reached a turning point, that we have seen our best days. But so said all who came before us, and with just as much apparent reason. ... On what principle is it that, when we see nothing but improvement behind us, we are to expect nothing but deterioration before us?