The Self Determination Theory...

This theory is devoted to the reinforcing of our intrinsic and naturally healthy tendencies, and was developed by Edward L. Deci (Professor, Clinical and Social Sciences in Psychology, University of Rochester) and Richard M. Ryan (clinical psychologist). Though these two developed the theory, it has been studied, researched and elaborated on by scientists all across the globe. 

Relating to Physical Activity...

It is in our very natures as living organisms to be actively pursuing challenging physical tasks. This allows us to become more skilled physically, and better capable of dealing with the demands set before us in this world. A life healthily exposed to physically/mentally taxing trials is more prone to progress, growth, and adaptation, all ideal outcomes that contribute to character and

self-motivation. This in turn will help lead one to greater success and reach higher goals. These goals could be looking more attractive or feeling better about oneself. Regardless of the endgame, the intrinsic motivators are the strongest driving forces that get us up off of our couches and into the world.

Not everyone is motivated in the same way...

It is not fully understood why intrinsic motivation comes to play