ok ... this will take a while.
First off: The Force is not a person, it is ... well just what it's name says. It is a natural force. Shapeless, directionless, purposeless. It is TWISTED into shape, form and purpose by force users who have the capacity to take this natural energy and twisting it to suit their OWN means. So the whole bringing balance to the force thing? IT DOES NOT MEAN ONLY JEDI! It means either equality between the forces of Jedi and Sith, or both groups extinct. That is how the force becomes balanced: either return it to it's natural state or let there be equal numbers of Jedi and Sith.
This entire theme has been played out like a cycle in the Star Wars EU, but I'm amazed how many people actualy fail to understand it ... in fact if you are to look at the inception of the Jedi and Sith orders you would find your answer to what Balance means in the star wars universe. Allow me to quote:
"The Je'daii Order was formed by colonists on the planet Tython 36,453 years before the Battle of Yavin. Tython has unique properties which allowed these colonists to master the use of the Force. The original Je'daii Order did not differentiate between the use of both 'Light' and 'Dark' Sides of the Force, but believed in the Balance of the Force; in other words, that a Je'daii needed to understand both the Dark and the Light, and use both accordingly.
Eventually, some members of the Order came to see the Dark Side as a path to power, causing a schism. These Dark Je'daii were defeated and exiled, with some eventually fleeing Known Space and heading to the Unknown Regions, where, as Sebastian_H mentions, they found the species known as the Sith. This species was extremely Force-sensitive and capable of providing many recruits to the Dark Jedi's cause, and so the Dark Jedi eventually came to call themselves Sith, after the people they established their rule over. Eventually, the philosophy of the Bogan - or Dark Side - came to be dominated by the ideology of the Sith, though there are plenty of Fallen Jedi who used the Dark Side without themselves being Sith; C'Baoth is an example of this.
After the exile of the Dark Je'daii, several Light Je'daii also left Tython, eventually establishing what came to be known as the Jedi Order. Tython itself eventually lost its central importance as both Light and Dark Jedi learnt to utilise the Force outside of its gravitational field, something that took a great deal of time to learn."
So you see they had right in the begining: both Light and Dark sides are needed for balance. It was those that chose pure Dark that started the imbalance, yes, but both are needed for balance. The original philosophy of the force has been forgoten and Force users have been divided and handicaped into pure light and pure dark. And so a great cycle of wars and death had begun in the galaxy: Sith Empire, Sith Wars, downfall and extermination of the Sith, Jedi ascension and the Republic, massacre of the Jedi by Anakin and the clones, The new empire, downfall of the empire and death of vader and the emperor ... and now a new trilogy will pick up in a universe where the Jedi once again dominate ... so I fully expect a carnage on Yavin and the destruction of the Academy. That is what logicaly follows the cycle. THAT OR Luke finaly breaks the cycle by readopting the original philosophy of the Je'daii and actualy restoring balance to the force.
See ... this cycle has happened so often in the Star Wars timeline there are even theories that the Force allways balances itself. So when there are too many Jedi they become weak, their senses are easier to cloud while the powers of the Sith skyrocket to match the ones of the Jedi: that's how one Sith can outsmart thousads of Jedi in the prequels. Sidious who manipulated an entire universe could not forsee that the rebels will win? Explainable now: Once the Jedi got wiped his powers got in line with the Jedi that were left. In a similar fashion: when the Sith were outnumbering the Jedi and controling much of the universe in the time of the Sith Wars: what logical sense does it make for the Jedi to just sudenly overpower and outsmart them? The Jedi beeing way fewer actualy got stronger: they became able to hold back Several Sith Knights and potentialy hundreds of Sith Padawans, while even regular people (republic troopers) could get kills on Sith force users when logicaly that doesn't make sense. Again: BALANCE. It's sad even the people writing the universe sometimes don;t get it because we are so used to the good guys winning when star wars was allways about how both light and dark need to win for balance to exist. THAT IS THE NATURAL ORDER OF THINGS! EVERYTHING HAS IT'S OPOSITE IN THE FORCES OF NATURE: good vs evil, life vs death, abstinence and control vs raw emotion and abandon, JEDI vs SITH. They need eachother yet they miss the point.
A theory that I find explains things is this: The Force is finite and it is split up evenly: half to the Jedi half to the Sith. When one side outnumbers the other they get weaker because the same power is split up among more people. That is why we see powers fluctuanting like that, that is why the rule is allways changing between Jedi and Sith: because they ignore this simple fact and constantly cause imbalance. BOTH SIDES. (fun aside: if this one is true then taking Luke from the end of A New Hope when he basicaly is the only Jedi in the universe and puting him in the prequels with all that potential power he has ... he could probably blow up a planet with the force (explains Vader's dismisive reaction to the Death Star planet busting power) ... which if i recal corectly was actualy done in the past)
And to the guy quoting that Grand Master Coven book labeled cannon: YOU DO KNOW YOU ARE JUSTIFING ONLY JEDI AS BALANCE USING A BOOK WRITEN BY A JEDI?! DON'T YOU THINK THERE ARE SOME ISSUES OF ... OH IDK: BIAS? CONFLICTING INTERESTS?
I mean of course lett's just act like we can just wipe out half of the force without realising we can't do that ... let's just say that we are the ones who deserve to live as we are superior and we bring balance to the force and all force users who disagree must be purged as they "create an imbalance" and "are not part of the Living Force, part of the cycle of life and death ... even though they live" ... reminds me of a certain famous mustache wearing austrian with a passion for art and a fetish for gasing jews and extending his right hand out ... YES! Grand Master Far Coven=Hitler! And she is a Jedi. Jeesus stop taking everything that hack Lucas vomits up as cannon ... that guy understands shit about what he created and has proven that time and time again.
As for the Jedi good guys ... you do know it's called JEDI Mind Trick right? They invented it. A method to use the Force to manipulate the minds and senses of individuals. Yeeeeeeeah ... I'm actualy on the side of we need the Sith to cull the jedi from time to time. Just for safety.