What causes steering to self-center?

[cscmc1]

I know this isn't a diesel-specific question, but you guys are so much better about answering my questions than are the other forums!

(On a W124 specifically,) what causes the steering to self-center after a turn? Is that a function of suspension geometry (camber, caster, toe, etc...), the steering box, a combination thereof, or something else entirely? I am trying to wrap my mind around how centering works.

Thanks!