One of the elements that makes this such a successful and hugely popular poem is the way that it addresses a fear that the majority of us experience at some point in our lives: what would happen if we died prematurely, before we have had a chance to fulfil our ambitions and dreams? This fear is addressed from the very first line of this sonnet:
When I have fears that I may cease to be
Before my pen has glean'd my teeming brain...
Keats, as a quintessential human being, hopes for, and expects the same chances to fulfill his calling (in his case as a poet) and to love the woman that he wants to be with. However, when he contemplates his own mortality, and the way in which his life is so fragile that it could be short, he is forced to reassess his thoughts and feelings about fame and stardom:
then on the shore
Of the wide world I stand alone, and think
Till love and fame to nothingness do sink.
The major theme of this poem therefore is our own delicate mortality, and how confronting this helps us to view certain central goals or aspects of our lives in an incredibly different way. Early death makes aspirations such as fame completely unimportant in the large scheme of things. What makes this poem so poignant is that Keats did die an early and unfortunate death from tuberculosis, leaving us with only a small result of what his "teeming brain" could produce.