The main theme of the poem is the brevity of life. This theme is touched on not only talking about the worries and insecurities of the poet, but also the frank observations of the knowledge he has that life cannot last forever. The poet seems to lament the shortness of life since he entertains hopes of experiencing so many beautiful things.
Some of the evidences that support the theme of life's short duration are the statements about the ceasing to exist or not living to fully experience all the magic life has to offer. Perhaps the clearest example of the poets idea of a fleeting life is when he states fair creature of an hour. This line demonstrates the true briefness of life from the poet's perspective.
It seems that faery power in this context is unconditional love, but that it also has a magical element to it. This part could be a consequence of the poet never having fallen in love or feeling incapable of finding such a love in the short time that he has available to him. It could also be a reflection of the beauty of a love that he can only imagine because it is out of reach.
The first quatrains of the poem, while laced with melancholy, speak of what the author hopes to gain from life and the experiences that he wishes to live. Even though he is aware that he probably will not have the chance, he maintains the desire to live them. The final couplet is characterized not by the acceptance of defeat but rather of the inevitable.
The overall message that can be taken from Keat's conclusion in this poem is that worrying over love and fame accomplishes nothing. He does not conclude that all effort is useless because beauty lies in ambition and desire. His fears do not make him an island because he feels connected to the Universe if only for an instant.
I would say that the answer is 'the poet uses dashes to slow readers down as they read through the poem', but that's kind of debatable. It doesn't fit any of the other answers, though.
