An interesting study from Denmark last year: http://www.sciencedaily.com/releases/2014/09/140916084909.htm
Here is the original paper that the Science Daily article refers to: http://www.jneurosci.org/content/34/37/12444.long
It is a very technical paper but what is says reveals that the simple explanation that Parkinson's is merely the result of too little dopamine in the brain is too simplistic. It is more to do with changes in the signalling patterns which utilise this neurotransmitter. It is all very complex.
To me this new investigation throws light on why symptoms and the response to levodopa medication varies so much from person to person and as the disease progresses through different stages.