When I have fears that I may cease to be (1)
What better way to start a poem about life's possibilities than by contemplating its end? Keats sure doesn't hold back on the drama – this first line makes it clear that the stakes of this poem are going to be high indeed.
And think that I may never live (7)
Wow, how many times can this guy postulate that he won't be around much longer? If you say it often enough, we'll start to believe it…
[…] then on the shore
Of the wide world I stand alone, and think (12-13)
It turns out that the best way to contemplate your death is to do it all by yourself. Death is not exactly an experience that you can share with anyone, so it's somehow fitting that contemplating death would also be a solo game.