But why one?

Yes, there are many stories. If we are writing it, we have a multitude of entries to choose from. In the preface to Obama's memoir, Dreams from My Father (p. vii, 2004 edition), he writes this:

[I] went to work with the belief that the story of my family, and my efforts to understand that story, might speak in some way to the fissures of race that have characterized the American experience, as well as the fluid state of identity -- the leaps through time, the collision of cultures -- that mark our modern life.

Beginning with that mission and that belief, he chose aspects of his story that particularly addressed race, identy, and culture.

He could also have said, "I went to work with the belief that my story could speak in some way to how growing up on an island affects how one lives in the world." or any number of entrances.

but in the end, it isn't up to us to write our stories, it's up to our loved ones.

Yup, especially if we're gone. Some people, though, want to make sure they have their say.

In any event, I'm not so much talking about writing our story (although perhaps it doesn't become a story until it is told or written). This meditation is more on taking the randomness we experience and finding a structure in it - seeing it in stages that take us from innocence, say, to experience; from a fragmented to an integrated self.

I speculate this: If there is no grand breakthrough and no change, if we keep destroying ourselves in the same way over and over again, it might be difficult finding an allegorical journey in our story or even a point.

This particular question arose from another one that I am working on called, "When did you smarten up?"