The reason being, motorcyclists know too well what it is like to be bullied by the bigger vehicles. The vast majority of the vehicles in North America, or northern Europe are 4 wheeled vehicles, with much more weight behind them than motorcycles.
If the two types (2 and 4 wheeled) of vehicle come into contact with each other, the vast majority of the time, it is the motorcyclist who gets the worst injuries. Motorcyclists are inclined to give smaller vehicles "their space", because the motorcyclists have their space invaded almost every ride.
I used to ride motorcycles when I was in my teens and twenties. It is a skill, like any other. The earlier you start, the more skilled you will become. Many people start well into their adulthood. Their lack of skill is somewhat offset by their less aggressive riding habits. Note motorcycles are 37 times as deadly per mile travelled, as a car is (for every mile a motorcycle travels, you have to travel 37 miles in a car to get the same risk of mortality).
http://www.iihs.org/research/fatality_facts_2009/motorcycles.html
If you are interested in motorcycles read the "ride reports" at "advrider", "pashnit", or "sportstouring.net". For example, this is one of my favorites :
http://www.advrider.com/forums/showthread.php?t=675841