The concept of Star Wars was that of there being balance in The Force. The Sith represent personal gain and a self-centered existence - where one forcibly subjects others for his or her own gain. The "dark side" of the force is often portrayed to be just that - acting in a selfish and self-serving manner.
The Jedi represent the opposite - a selfless devotion that all but erases one's individuality.
In some sense, the Jedi are the defacto good-guys, as they care about the balance of the force and submit to the concept of being guided by "the force." Without the Jedi (or someone else 'following the call of the force') - there would be only Sith.
But the balance requires there to be selfish ambition as part of it. It's a sort of Yin and Yang concept; or it could also be looked at as a sort of 'Turbulence Theory' - drawing a parallel to physics where fluids (or gasses) with imbalanced heating will become turbulent with certain regions having much higher energy densities than others (causing exchanges of energy between these imbalanced regions that, in planetary environments, results in thunderstorms). An active and healthy system will have conflict that arises from the differences in ideologies.