Are you talking about a body in orbit? How the orbit gets smaller (decays) as the ‘moon’ decelerates? Basically why does gravity pull things in?

In general the 'least resistance ’ idea usually comes down to the first and second laws of thermodynamics - 1. Energy can never be created or destroyed in a closed system, just transformed and 2. Everything tends towards maximum entropy minimum energy (rocks don’t roll uphill). These are often taken as fundamental and all the why’s come from them. Why? Entropy baby.