What is the narrator’s motivation for telling this story?

The narrator's motivation in telling this story is to unBurden his heart and soul before his death. we can find this in the first passage of the story. To quote,

"But to-morrow I die, and to-day I would unburthen my soul. My immediate purpose is to place before the world, plainly, succinctly, and without comment, a series of mere household events."