Neurological explanation for differences in what motivates runners?
Your question is really broad and can encompass libraries. However, to add my 2-cents worth regarding 


  specifically what is the [...] psychological root cause for individuals being motivated by different sorts of things...


I would say that a lot of this has to do with external factors, and specifically external sources promoting exercise to be 'healthy', including peer pressure and multimedia.

To pin this global effect on our way of thinking down to  


  [the] specific[...] neurological root


is not really feasible in a single answer in theSE framework, as it is just way too complex to grasp on a single web page in a Q&amp;A forum I'm afraid. But the between-subject differences can, at least partly, be explained by intersubject environmental differences.  

Reference
- Brody &amp; Inoue, Issues Innov Nursing Practice, 218-27