I recommend reading Ian Morris' book Why The West Rules - For Now. He discusses this topic in a few chapters.
Although Ming Dynasty China had ships which could cross the Pacific and sail around the entire world, the government ministers chose not to. (The Ming Emperor was 12 years old at the time, so the government mandarins would have been making the decisions, similar to the constitutional monarchies of today although there was no elected Parliament). Sending ships across the world would have put massive amounts of men and money at risk, and what is the upside? All the resources the country needed were either in the territory it controlled or could be acquired through overland trade routes. Sending expeditions to faraway lands was a high risk, low return venture, to borrow an investment phrase. Finally, consider that crossing the Pacific is way longer than crossing the Atlantic.
Contrast this to the Atlantic facing nations of Western Europe. Resources were more contested, given the fact that Europe of the time was fragmented into many nations and they were at war with each other for most of the time. A western European nation state was in control of a smaller amount of land than the imperial Chinese dynasties and could not muster the same resources. Therefore, sending ships across the world had a much higher potential for a big payday. They had to do it, there was no choice. Even living in Britain today, go to the grocery store and look at how much stuff on the shelves comes from other countries. Also, see my earlier comment regarding how the Atlantic is a lot smaller than the Pacific.
Western Europe was the ideal place to sail around the world, it is in the centre of the half of the earth which has the most land. See this Wikipedia article:
Finally some food for thought: Ian Morris mentions that the monarchies in Britain and the Netherlands were weaker than the Spanish one. This meant that the businessmen and merchants in northwest Europe had more of an incentive to finance overseas trade whereas Spain used seafaring trade voyages to enrich the government.