A memoir takes a certain amount of arrogance to write….One must think one's life is important or interesting enough to palm off on an unsuspecting public. At least fiction writers have the pretense that their work has more to do with their characters than with themselves. Still, I doubt you'd find much of a difference between a memoir writer and a fiction writer in the humility department.
Or maybe memoir writers tend more toward exhibitionism, are more willing--eager, in fact--to slap their cards on the table and squawk, "Read 'em and weep." The fiction writer, cagier, plays his hand close to his vest, pretends he knows how to bluff.
If you write your life down on the page, beginning with "I was born in…" and ending with, "As I pen these immortal words, I gasp my last breath," what you've probably got is a self-indulgent autobiography, not a memoir. A memoir usually deals with a portion of one's life--say, childhood--not the life in its entirety.