And summer’s lease hath all too short a date (4)
The basic idea here is simple: the time summer occupies during the year is too short, as it has to give way to the other seasons. But "during the year" isn’t the only time frame one can look at. In fact, the bigger the span of time you look at, the more summer there is, since summer happens every year. In a sense, then, summer actually is immortal, even though the poet will go on to contrast his beloved’s "eternal summer" with the natural summer.
And every fair from fair sometime declines, By chance, or nature’s changing course untrimm’d (7-8)
We considered calling this theme "Fate and Free Will," because the main issue the poet has with time is that it imposes a fate on people, just as it does nature. Like summer, which can’t last all year, beauty can’t last an entire lifetime, as time will eventually catch up with you (or you could go the plastic surgery route). It is only through something eternal like poetry that man can escape, to any extent, the fate imposed by time.
But thy eternal summer shall not fade, … When in eternal lines to time though grow’st (9 and 12)
Nothing new here, but it’s worth noting the repetition of "eternal." This isn’t a poem that tosses words around lightly, so repeating a word means it’s very, very important.