Well, here's the thing about Keats' title: it's not really a title, just the first line of the poem.
Think of this poem as the scribblings that you'd doodle on your notebook in math class, or a note that you'd pass to your friend while the teacher's looking the other way. Okay, so most notes that you write probably aren't in iambic pentameter. And they probably don't use super-flowery imagery about fairies and ripening grain. We get that.
Here's the thing, though: Keats' world was full of fairies and stormy seas and metaphors of all shapes and sizes. That was just his life. when he penned this little poem and sent it off to his best friend, he didn't take the time to write a title at the top of it. "When I have fears that I may cease to be" is a message to a close friend. No title needed.
Years after Keats' death, the editors and publishers who finally got around to printing Keats' work needed some way to distinguish this poem from the others, so they snagged the first line and called it a title. Since we're talking about the title, we should point out that not titling Shakespearean-style sonnets is, after all, precisely what Shakespeare did himself. At least, Keats was in good company.