What were the economic forces behind Spain and Portugal's desire to colonize the Americas?

There were several economic forces behind Spain and Portugal's desire to colonize the Americas. First, Europeans were desperately searching for sources of gold and silver. Gold was actively traded on the Silk Roads and Trans-Saharan trade networks, and silver was mined from the East. Unfortunately for Europeans, there were no large sources of gold or silver in Europe. When Europeans arrived in the Americas, they saw evidence of gold and silver—Columbus, in his journal on Saturday October 13, 1492, wrote about how the natives were wearing pieces of gold as jewelry:

I was very attentive to them, and strove to learn if they had any gold. Seeing some of them with little bits of this metal hanging at their noses, I gathered from them by signs that by going southward or steering round the island in that direction, there would be found a king who possessed large vessels of gold, and in great quantities.

Columbus's journal was a letter to the king and queen of Spain, persuading them that his journey was a success because he found gold. He mentions it several times, and in his subsequent explorations, he searches for more gold. Francisco Pizarro and Hernan Cortes, in their conquest of the Inca and Aztec (respectively), also bargained for and pillaged gold from the respective empires. So as Columbus's journals show, the Spanish and Portuguese were actively seeking out gold and silver, and once they found it, they wanted to colonize the Americas to extract it.
A similar economic reason for Spanish and Portuguese colonization of the Americas was the location and the land itself. Once Spanish and Portuguese explorers conquered South America and various Caribbean islands, they discovered that they could plant sugarcane on this land. The plantations that populated these islands made Spain and Portugal very rich and encouraged further colonization. The Spanish and Portuguese also profited heavily on rice, cotton, and in later years, coffee.
A third economic reason for Spanish and Portuguese colonization of the Americas was to find a spice route between Europe and India. Before Columbus sailed west and landed in the Bahamas, the Portuguese were seeking a naval route to India around Africa. Once the Christian city of Constantinople fell in 1453, Europeans were weary of land trade and how expensive it may be now that their trading hub on the Silk Roads was a Muslim city. Anxious to find their own trade routes (and therefore make more profits by directly importing valuable spices from the East), the Portuguese, and later the Spanish, launched expeditions in search of a water passage to India that could bypass the usual routes. Eventually, with the help of explorers like Bartholomeu Dias and Vasco da Gama, a way around Africa's southern tip (the Cape of Good Hope) was discovered, and a direct trade route from Portugal to India was established. Some explorers headed west to find a trade route (like Columbus). It was only confirmed that the Americas were a new continent group in the early 16th century; initially, it was thought that the Americas were India or China, hence the name "West Indies," colloquially used to describe the Caribbean islands.
http://www.let.rug.nl/usa/documents/before-1600/extracts-from-the-journal-of-columbus.php