Cynicism
Sir Walter Ralegh was a guy who lived large. He took big risks and those occasionally reaped big rewards. The ones that didn't work out, though, really didn't work out and ended up getting him into a lot of trouble. Just a few short years after enjoying favor and popularity with the royal family, Ralegh found himself locked up in the Tower of London and ultimately executed upon charges for treason.
While he certainly had lots to be cynical about later in life, it's interesting that scholars credit most of his poems to an earlier period where he seemed to be in good spirits and, by most standards, very successful. The cynicism we see in "The Nymph's Reply to the Shepherd" crops up over and over again, however, as we continue to see Ralegh portraying Time as a ruthless force propelling us towards death and life as nothing more than a series of falsehoods and trivialities waiting to disappoint us. But don't take our word for it. Check out some of these other poems by Ralegh and see for yourself: "What is Our Life?," "The Lie," "Farewell, False Love," and "Nature, That Washed Her Hands in Milk."