Quite true. The main reason I like this model is that it reminds one (if you use it) to be conscious of two things in your relationship :
1. To keep your own individuality - which is what we all want - to be able to be completely and fully ourselves with others, and for the other person to be fully genuine/themselves with us; and
2. to be conscious of : the effort it takes to grow a relationship and keep it alive; the importance that we place on the relationship effects how 'large' it grows.
Outside influences will of course effect the relationship, but my model of course looks more at an individuals contribution to it.
Let me put it this way - we, to a very large extent, are responsible for who we have in our lives (the number of people and the quality of them) : our friends, our loved ones, our family, our associates that like socialising with us etc. Cultures and other influences will of course play a part, but as a generalisation, anyone who takes responsibility for their own lives should comprehend this.
Perhaps if I compared it to hypothetical models of the human brain - with my model, lets compare it to a model of the 'core / primal' areas of the brain that drive us. Describing this area in a model would be accurate for the core, but not for the entirety of 'being human' (for we all know we are much more complex than that)....yet if you modeled 'higher' models against it (the core model), you would likely gain a greater understanding of how the 'higher' model fits into human life by running it simultaneously alongside the 'core' model (higher models often overlook genetic drives)
In other words - I see the model I use as a good starting point, and after the add complexities in, as a reference point.
It's also useful in that it generates the question 'what am feeding this relationship (what am I contributing to it)' and 'am I being respectful of my individuality and also being respectful of the others individuality' - which are questions we often avoid when difficulties arise. It also helps comprehension of when the other party is making no effort to feed the relationship, what is actually happening. Etc.