1) Political Factors: In 1453, the Orthodox Christian Constantinople fell before the Ottoman Turks. The control of the trading routes to the East was now controlled by the Turks, who charged heavy duties from the Italians for the trade. There was, thus, the urge to find new trade routes to the East. Both Spain and Portugal were lucky to have strong rulers and innovative sailors who took up this task. Innovators such as Prince Henry of Portugal established training schools for the navigators, and thus encouraging large-scale-sea-borne explorations. Spain, on the other hand, was brimming with confidence after being victorious in several contemporary military conquests led by Cortez and Pizzaro. These victories gave a boost to their ambitious spirits and a further kick to venture new sea routes to new colonies.
2) Religious Motives: The missionary zeal to convert more people to Christians was a primary reason to look for new lands.
3) Economic Factors: The zeal to convert people to Christianity soon encouraged the political and economic interests of the Europeans. The crusades against the Turks, for instance, was based on religious motives but it, in turn, also increased the demand of Asian goods in Europe, thus, encouraging trading between the two lands. For the Portuguese, West Africa was an unexplored land rich in spices and gold. Thus, their colonial adventure began with the attack on this rich land of resources in 1415.