I've compared R's to celestial bodies under the effects of gravitation and the Laws of Motion. Like binary stars. A couple is in orbit with each other -- some orbits are circular and stationary, others (most) are more elliptical in nature. Thus there is an aphelion and perihelion to our orbit with our spouses. If nothing else changes, the orbit should be maintained, no matter how eccentric the orbit might be. This would approximate "the high and low points" you alluded to. However, there are the gravitational influences of other objects that tend to disturb the stability of the orbit.
My W was influenced and torn from her orbit with me because another gravitational body came into influence right at the worst point, near aphelion. I guess she just didn't have as much gravitational mass as I thought.
