Among other things Parkinson’s disease involves a question of balance – not just physically but also chemically. In this case the balance between dopamine and acetylcholine.

Acetylcholine controls-transmits the smooth muscle impulses. Depleted dopamine results in irregular stop-go impulses.

One of the easiest explanations is provided at the site below: The diagrams are basic. They show the healthy/normal nerve impulse sequence on the left and the Parkinson’s disease sequence of events on the right:
http://www.holisticonline.com/remedies/Parkinson/pd_brain.htm

There are 5 categories of drugs used to treat Parkinson’s disease:
Dominergic – the antagonists, L-Dopas, which attempt to restore the levels of dopamine in order to slow/stop/reverse the symptoms of PD.
Dopamine agonists: which try to mimic the natural dopamine and are taken with the antagonists for better result since these wear off increasingly.
COMT inhibitors: which are also taken with the levadopas to prolong the good effects.
MAO Inhibitors: which can also be used with the L-dopas but are often taken also – They work to stop the breakdown of dopamine by monoamine oxidase B – if the breakdown of dopamine can be slowed, the dosage of the L-dopas should be reducible in patients taking both.
and
Anticholingerics: By blocking the acetylcholine the goal is a compensatory balance of the mis-firing of the neurotransmitter dopamine and the normal firing of the acetylcholine in order to relieve a variety of movement issues in PD.

In PD it is not only the loss of dopamine neurons which bring about the Parkinson’s symptoms but also a loss of norepinephrine – rather than try a wordy explanation, here is a really good one:
http://www.restoreunity.org/inhibiting_acetylcholine_parkinson.htm

Another article to read about the begining of the question of why the dopamine breaks down – there are links to the technical articles:
http://parkinsonsfocustoday.blogspot.com/2009/05/parkinsons-disease-triad-for-dopamine.html

The regulation of dopamine/dopamine balance is the focal point of most existing treatments because it is the 60-80% depletion which triggers the symptoms which send the patient to diagnosis.

About dopamine:
http://scienceblogs.com/developingintelligence/2007/02/dopamine_for_dummies.php
And I just found this one – not very scientific but it raises a good point:
http://www.sciencedaily.com/releases/2008/08/080808160331.htm

The treatments are just that – treatments to delay the progression, minimize symptoms – they are not the cures. There may be cures in GDNF or just good treatment. There is surgery to minimize dosages but the disease is still there.

Just keep in mind that Parkinson’s disease isn’t all about acetylcholine and dopamine. And the answers may lie in finding whay the alpha-synuclein "gums up" or…..

You can go to medhelp.com and they have different forums there when
people that actually have this disease will answer your questions.