The speaker of Sonnet 60 seems pretty bummed out. From the very beginning of the poem, with its imagery of the waves endlessly moving forwards to dash themselves to pieces on the shore, all the way to that last line about time's "cruel hand," this is one down-in-the-dumps dude. Line 4 conveys special bitterness, in the way it emphasizes the effort ("toil") the waves put into struggling ("do contend") to make their way forward. "Why do they bother?" the speaker seems to be asking. Now there's a doozy of a question.
Over the course of the next two quatrains, the speaker seems to go from depressed to angry, as he talks about all the beautiful things that time destroys. And yet, it turns out that the speaker also has hidden reserves of strength—as the couplet reveals. In these final lines, the speaker is able to turn his anger at the destructions of time into a steely determination to oppose time with his poetry—and to continue singing the praises of the person he is talking to. So it's not all bad. This guy may be the tortured-artist type, but he might not always think his glass if half-empty.