Mental Factor which Unites Other Factors If I think of the Noble Eightfold Path for example, or even the Abhidhamma traditions, I feel mental factors are clearly related, but noted as separate. 

In sum, my question mainly is: What conditions cause mental factors/qualities to aggregate, i.e. lead to the development of others, and what conditions cause them to remain separate? Is there a unique condition (e.g. a mental factor) that unites others in the mind?

In non-Buddhist sources, strong pleasure as in addiction seems to channel the mind towards doing but one thing, to exclude in a way causing shame. But positive emotions such as joy tend to broaden one's perspective and capacity of action. A researcher on this says: 


  But as the sun rises in the sky, things begin to change. Your blinders
  around your face begin to open and your world quite literally expands.
  You can see more. Your world is larger.
  
  Just as the warmth of sunlight opens flowers, the warmth of positivity
  opens our minds and hearts. It changes our visual perspective at a
  really basic level, along with our ability to see our common humanity
  with others.


Can this be related to this question?

Thank you
Your questions sounds somewhat similar to Visākha's question to the nun Dhammadinnā in MN44:


  “But ma’am, is that grasping the exact same thing as the five grasping aggregates? Or is grasping one thing and the five grasping aggregates another?”
  
  “That grasping is not the exact same thing as the five grasping aggregates. Nor is grasping one thing and the five grasping aggregates another. The desire and greed for the five grasping aggregates is the grasping there.”


In summary, desire and greed perpetuate the five grasping aggregates as identity. The dissolution of identity (your "condition that causes them to separate") is another one of visakha's questions later on in the sutta:


  “But ma’am, how does identity view not come about?”
  
  “It’s when an educated noble disciple has seen the noble ones, and is skilled and trained in the teaching of the noble ones. They’ve seen good persons, and are skilled and trained in the teaching of the good persons. They don’t regard form as self, self as having form, form in self, or self in form. They don’t regard feeling … perception … choices … consciousness as self, self as having consciousness, consciousness in self, or self in consciousness. That’s how identity view does not come about.”