Each of us, in fact, writes his own history, by cherry-picking those bits of pre-existing history cherry-picked and filtered by others, that support our world view.

From extensive study of the JFK assassination (my work is available at patspeer.com) I came to see how people investigating the case, from day one, saw Oswald either as a "patsy"--an ordinary guy put in extraordinary circumstances--or as a psychopathic loser out to make a name for himself.  It's clear that those in the upper echelons of society-people who think of themselves as self-made or as "deserving" of their success, resent that a man as poor as Oswald, a Marxist, would have people believe in his innocence.  To their minds, Oswald's motive is obvious: he hated himself and his failed life and thought his only chance at making a name for himself was by killing the President.

To others, including myself, this is ludicrous. Of course, Oswald MAY have wanted to kiil Kennedy for personal reasons, but nothing in his behavior indicated as much. He certainly never bragged about killing Kennedy. He even told the cops that killing Kennedy was stupid because Johnson would be no better.  

Another factor determining one's view of the assassination is one's respect for expertise and authority. Those holding that Oswald did it most always defer to "experts" of one sort or an other, and attack the expertise of the nay-sayers.  If you challenge the experts on some of their conclusions, and even prove these experts incompetent or deceptive (as I myself have done in the videos at patspeer.com) they will not respond to you with facts, but attack your right to question the authority and initegirty of the experts.  They will do this even when you show something as simple and obvious as that a doctor testified with his exhibit upside down.

Bottom line: official history is the cumulative guesswork of those with a platform to broadcast their guesses.

Perspectives on history change not because people change their minds so much as that those of one perspective die off and are replaced by people exposed to different platforms.