Why did exploration happen in the Exploration EraWhy did exploration happen?
The "three G's" were certainly the motives of individual explorers, and I agree the gold and glory were probably the most important. Another question to ask, though, is why exploration became possible or desirable. This has to do with the geo-politics of the spice trade, the decline of Chinese naval power in the fifteenth century, the new cosmopolitanism stemming from the Crusades and trade, the emergence of new financial instruments (a result as well as a cause) and the development of new technologies that made journeys across the Atlantic possible. Overall, it's important to note that the Portuguese, in particular, had been exploring the coast of Africa for almost a century by the time the "Age of Exploration" really kicked off. The knowledge they gained was an important factor too.
Like #2 and #3, I would say the "Three G's" are the broadest reasons for exploration. I think every student must learn that mnemonic device. I know I did when I was in school, and when I taught U.S. History and World History, I used it with my students too!
If I had to rate the three reasons in terms of which one was the biggest motivating factor, I would definitely say gold. Trade, expansion, and colonialism drove exploration; particularly because the companies and countries who financed the voyages were hoping for large returns in terms of monetary compensation.
The thing you need to understand is that the people doing the exploring did not call the time in which they lived "The Exploration Era." That title was created years later by historians looking back and realizing that motivations, conditions, and advances in knowledge all came together to support a burst of exploratory activity in a relatively short period of time.
We typically say that it happened for "God, Gold, and Glory." In other words, it happened because people wanted to get rich, because they wanted to gain glory and power for themselves and their country, and because they wanted to spread Christianity. Most people would argue that gold and glory were much more important than God in this process.