God, Gold and Glory was indeed the avowed aim of the Spanish Conquistadores; yet they were only one of several groups of Europeans who engaged in European expansion. A number of other factors were at work. A large factor was the capture of Constantinople by the Turks in 1453. Europeans had previously purchased spices and Oriental goods there, but had no desire to trade with Islamic "infidels." Many nations wished to find their own route to the East and avoid Constantinople. The first to successfully do so were the Portuguese, when Vasco da Gama reached India. He informed the Indian emir that he came for "Christians and Spices."
The European Renaissance was also a major factor. The increased emphasis on experimentation, knowledge and learning led to superior methods of navigation and ship building, many of which were borrowed from the Muslims. This included use of the Astrolabe, the lateen sail, and the Compass. This made sailing over long distances more feasible.
The consolidation of nation states in Europe meant that monarchs now had the means to fund overseas exploration. Also some exploits were privately funded (Christopher Columbus privately funded two of his ships, only the Sana Maria was funded by Ferdinand and Isabella) most were pursuant to royal charters. John Cabot (Whose real name was Giovanni Caboti) sailed for England, even though he was Spanish. Henry Hudson sailed for the Dutch even though he was British.
Exploration along the coast of Africa was spurred by the search for a mythical Priest King named Priester John. When Prince Henry the Navigator sent expeditions down the Atlantic Coast of Africa, they carried a letter from the Pope addressed to Priester John and written in Latin asking for his help in expelling the Muslims from the Holy Lands.
Finally, Marco Polo's book, A Map of the World, inspired great interest in exploration and the search for riches. Although many portions of his book are suspect, and there is even substantial doubt as to whether he actually existed, his book did create tremendous interest in Asia. Christopher Columbus owned a personal copy of the book with notations in the margins in Latin.