We human beings are so unique that what works perfectly well for one individual may not for someone else. While some of us are happy operating at a surface level, others might crave to make more soulful bonds where we can be ourselves and say what is truly on our minds.
That level is set by the person with the least amount of comfort with vulnerability and it typically stays there unless they experience change. I have witnessed couples where interaction is so mundane and yet it works for them which is what counts. It is therefore the gap in the desired level of connection that becomes the metric for health.
Profundity isn't what is always sought but instead compatibility which is the best foundation to build on. A connection that doesn't have that can burn out once the pheromones have expired and reality settles in. There is also the unfortunate reality that love is often not balanced; that gap in the level of investment revealing itself only when we reach veritably stormy seas.