Because If feelings like these were confined to the present – what we are doing now to address our current circumstances - the challenges of living would be simpler.  Indeed, Whitman was a champion of that fully lived moment, when we embrace concrete, sensuous existence.  But unhappiness also centers on past occurrences, such as an ill-chosen act or a disgrace that can never be rectified.  And we are tormented equally by events to come, not just the “revenge of the future”