The need for new trade routes to the East, the desire to spread Christianity, and competition among European countries prompted the start of European exploration in the 15th and 16th centuries.
In the 15th and 16th Century, Navigation and Exploration were at their peak, and European Countries were assembling more and more colonies to their Empires.
As stated above, Competition among European Countries was a huge factor. Every country wanted to have an edge over other countries, explore private trading routes, provide their countries with slaves acquired in the colonies, as well as exploring the natural resources of such colonies.