Well, I was gonna cut and paste from a couple very loooong essays but that's just a waste of bandwidth.
Here is one link, http://www.ianchadwick.com/motorcycles/whyride.html and if you are good with google you can find many more.
For me, it boils down to this:
In a car, it is just like watching more TV.
Images seen through glass with metal borders.
On a bike, I am
IN the world. I pass through it, it passes around me.
I am involved.
I am.

Kris