I can give you a little bit of the history:
In Shakespeare's time, thee/thou/thy/thine AND you/your were used.
You/Your for formal address* and also for plural (if talking to more than one person).
Thou/Thee/Thy for informal address, for when you're talking to one person that you know, love, or look down on.
Then I guess people just gradually stopped making that distinction, and decided they'd be polite to everyone, thereby depriving the formal address of its formality.
*if want more specific information on formal versus informal address, look up information about the Spanish "tu" and "usted", or the French "tu" or "vous".
-ism: doctrine, system of beliefs [<Greek -ismos to separate motions< cosmos, to cause motions]
The evolution of living languages, through human carelessness with words, is the cause of all evolution.