My main question is: how it is achieved that a person is interested and excited in their work and want to generate improvements, beyond its formal obligations ?.
There's a whole field dedicated to this: Positive Organizational Behavior (or Positive Organizational Scholarship). Google it and you'll find plenty of books and articles.
As for a theoretical position, a good place to start is self-determination theory by Ryan and Deci:
Ryan, R. M., & Deci, E. L. (2000). Self-determination theory and the facilitation of intrinsic motivation, social development, and well-being. American psychologist, 55(1), 68.
a very relevant topic of research on this is emotion, motivation and work attitudes/behaviors.
Another relevant sub-field/topic is work/organizational commitment.
See below a few relevant papers
Levine, E. L., Xu, X., Yang, L.-Q., Ispas, D., Pitariu, H.D., Bian, R., Ding, D., Capotescu, R.. Musat, S. & Che, H.S. (2011). Cross-national explorations of the impact of affect at work using the State-Trait Emotion Measure (STEM): A coordinated series of studies in the United States, China and Romania. Human Performance, 24, 1-38.
Yang, L.-Q., Simon, L., Wang, L., & Zheng, X. (In press). To branch out or stay focused?: Daily affective shifts differentially predict organizational citizenship behavior and task performance. Journal of Applied Psychology. http://dx.doi.org/10.1037/apl0000088
Johnson, R.E., Chang, C.H., & Yang, L.-Q. (2010). Commitment and motivation at work: The relevance of employee identity and regulatory focus. Academy of Management Review, 35, 226-245.
There are thousands of articles on job satisfaction. The essentials: personally interesting and challenging work; high pay; fair pay and pay systems; managers who are competent, honest and care about you; competent, honest coworkers; safe and pleasant location; good org. leadership; chance to grow; good benefits.