Any number of factors led to European Exploration, dating back to the Crusades. The Crusades were the first exposure of Europeans to Eastern products which were in high demand in Europe, primarily sugar. Additionally, Marco Polo's book, A Map of the World contained glowing (if inaccurate) descriptions of the wealth of Asia. An example:
It should be understood that the sea in which the Island of Zipangu [Japan] is situated is the sea of CHIN, and so extensive is this eastern sea that according to experienced pilots and mariners, who should know, it contains no fewer than 7,440 islands, mostly inhabited. It is said that every one of the trees which grow in them gives off a fragrant odor. They produce many spices and drugs, particularly aloes, and much pepper, both white and black.
Additionally, the rise of Nation States in Europe led to increased revenue for Monarchs who were able to finance European voyages of discovery. Notable here was the marriage of Ferdinand and Isabella which united Spain into a united country.
A final factor was the fall of Constantinople to the Turks on November 27, 1453, Constantinople had been the primary market for Asian goods; however after it was taken by by the Turks, Europeans searched for ways to circumvent the Muslims.
The Crusades and the end of the Middle Ages created the conditions needed for the Age of Exploration to happen. When the crusaders came back from the Middle East, they brought with them silks and spices and other goods from the Far East that people in Europe began to demand. With the end of the Middle Ages, there was a corresponding increase in trade. Huge fortunes, especially by Italian merchants and bankers, were made from this trade with the east. Also with the end of the Middle Ages, nations began to develop and power and wealth became concentrated in monarchs throughtout Europe. These monarchs saw there was money to be made in trade with the east and began to finance trips of exploration. Most of the voyages of discovery made during the Age of Exploration were in reality attempts to find faster and cheaper trade routes to the Far East.
Lots of things led to the age of exploration. Let me mention just a couple:
- Improvements in technology. When Europeans got better ships (caravels) and better navigational equipment, they became more able to go out exploring.
- Changes in mentality. The age of exploration came out, to some extent, of the Renaissance. The Renaissance was a time when people (among other things) became more interested in knowledge. They wanted to find out what was out there beyond the horizon in ways that they had not really wanted to before.
So these are two causes -- a greater desire to explore and more technology that would make it possible.