Written like a Shakespearean sonnet, this poem is sometimes given the fuller title "When I have fears that I may cease to be." The central theme is the fear of death. For Keats, he fears death for the reasons most people do but he also fears he will die before he's had a chance to establish himself as a timeless poet:
Before my pen has glean'd my teeming brain,
Before high piled books, in charactry,
Hold like rich garners the full ripen'd grain; (2-4)
"Charactry" means characters (letters) of the alphabet. He also fears dying before he can experience love, "Never have relish in the fairy power/ Of unreflecting love;" (11-12). Keats' father died when he was eight and his mother died when he was in his teens. So, he was very aware of how fleeting life can be and perhaps believed that dying young was in his blood. Keats always feared a young death and unfortunately and prophetically, he died when he was 26. Keats feared that he would not have enough time to experience love (Fanny Brawne would be the love of his life) and not enough time to develop his art, "To love and fame do nothingness do sink."