The primary reason for colonial expansion in the 15th and 16th centuries was the quest for new trade routes and access to valuable resources. European nations, particularly Spain and Portugal, sought direct trade with Asia for spices, silks, and other goods, which were highly coveted. This led to explorations by figures like Christopher Columbus and Vasco da Gama, aimed at finding shorter routes to Asia. Additionally, the desire for land, wealth, and the spread of Christianity motivated these explorations. The outcomes included the establishment of colonies, trade networks, and significant impacts on indigenous populations, eventually shaping the modern world.