Why didn't China colonize other parts of the world?

From the fall of Rome to the age of Discovery, China was the most technologically advanced country in the world. Their ships were far superior in construction, durability, technology, etc. than European ships. But from the unification of China in the early-200’s BC until European countries became the dominant powers, China did not sail around the world setting up colonies and expanding their empire. Why not?

The Rise & Fall of 15th Century Chinese Seapower:
http://library.thinkquest.org/20176/chengho.htm

I suppose you could write that the Chinese are by nature more peaceful than Europeans…however China has a history at least as brutal as that in the West with civil wars, famines and massacres, just like the rest of the world. So why did not China simply expand its power beyond its borders? Is it as simple, as they had everything they already needed?