What Spain was really interested in was to find a sea route to the orient (Asia) and principally India. At that time, all the various spices, silk and many other items came from Asia and were extremely valuable. The existing trade route was by land and it was long, arduous and dangerous. Spain wanted to compete with other European countries in this highly profitable market (Abernethy, David (2000). The Dynamics of Global Dominance, European Overseas Empires 1415–1980. Yale University Press).
At that time Portugal had asserted itself as a dominant sea power so Columbus first approached the Portuguese monarch suggesting that he could reach India by sailing west rather than southeast (around Africa) as was the Portuguese plan. The Portuguese already knew that there were other lands that could be found by sailing west - they had already begun colonization of the Azores archipelago in the North Atlantic and there were reports of other lands much further out west by fisherman and other Portuguese explorers (Boxer, Charles Ralph (1969). The Portuguese Seaborne Empire 1415–1825. Hutchinson).
The Portuguese were excellent sailors and cartographers and were skeptical that these lands were any part of Asia so they were dismissive of Columbus plan -The king submitted Columbus's proposal to his experts, who rejected it. It was their considered opinion that Columbus's estimation of a travel distance of 2,400 miles (3,860 km) was, in fact, far too low (Samuel Eliot Morison, Admiral of the Ocean Sea: The Life of Christopher Columbus,(Boston: Atlantic-Little, Brown, 1942). Reissued by the Morison Press, 2007).
In 1488, Columbus appealed to the court of Portugal once again, and once again, John II invited him to an audience. That meeting also proved unsuccessful, in part because not long afterwards Bartolomeu Dias returned to Portugal with news of his successful rounding of the southern tip of Africa (near the Cape of Good Hope). With an eastern sea route to Asia apparently at hand, King John was no longer interested in Columbus's far-fetched project, but offered him a captain-ship in their fleet. Columbus declined and approached Spain with his proposal and plan which after much deliberation was eventually accepted.
Columbus sailed west and reached the Caribbean Islands and thought that he had landed in some islands off the coast of Asia. He held to this belief until the day he died. ( Davidson, Miles H. (1997), Columbus then and now: a life reexamined, Norman, OK: University of Oklahoma Press).
He simply underestimated the size of the earth. In theory he was right. By heading west from Spain all around the globe he could have reached India, but he would have to have crossed all of the Atlantic and all of the Pacific Ocean and would have perished at sea. Columbus landed in the Caribbean in 1492. By 1498 the Portuguese had already arrived in India following their planned route by sailing southeast. Over time they established several colonies in Asia and overwhelmingly dominated the trade route with Asia for over one hundred years >(Russell-Wood, A.J.R. (1998). The Portuguese Empire 1415–1808. Johns Hopkins University Press).