What causes motion?

[geordief]

Is motion just the natural order of things  and so we are really asking what changes motion?

 

In any system is there a process of seeking a balance  and this process is unattainable so motion continues?

 

And why is there a favoured direction of motion (along geodesics)?

[swansont]

Motion is relative and any inertial observer can say they are at rest. Nothing is required to maintain motion.

As to what changes motion, it’s acceleration, and acceleration requires a force.

A geodesic isn’t a direction, per se. Symmetry tells us there is no preferred direction. But the answer is the principle of least action.
 

https://en.wikipedia.org/wiki/Geodesics_as_Hamiltonian_flows#Geodesics_as_an_application_of_the_principle_of_least_action