Why did Christopher Columbus want to go to Asia?

Throughout the Middle Ages and the Renaissance, European traders traversed the relatively safe Silk Road to Asia in order to procure the silk and spices that were in high demand. By the mid-fifteenth century, however, this route had become quite dangerous, and rather than give up this highly lucrative trade, merchants began thinking of looking for a water route to Asia.
Christopher Columbus was a mariner from Genoa who had first set sail as a young boy on merchant ships, some of which sailed part way down the coast of Africa. He was also very well-read in such areas as history, astronomy, and geography, and was well aware that educated people understood that the world was round. As his skills as a navigator and explorer increased, he decided that he wanted to lead an expedition to the west in order to find a waterway to Asia. He attempted to get backing for such an expedition in Genoa, Venice, England, and Portugal, but he was turned down. Finally, the rulers of Spain, Isabella and Ferdinand, gave him their backing and promised him that he would be made an Admiral of the Seas and a viceroy of any lands he discovered, and he would get a percentage of the profits of the spice trade if his expedition proved successful.