[morrow]

Not to get too analytical or philosophical, but I think it's because clocks normally represent time cyclically, as a repetitive process counting and displaying a sub-set of an ethereal infinite time set. Whereas this clock visualizes time spatially in known, limited, diminishing quantities.

The depressing part is either that this forces you to realize or reminds you that time is, in fact, limited -- or that it tricks you into thinking time isn't infinite, depending on your philosophy.