They colonized the Americas to search for needed minerals such as gold. Then as they started to settle, they introduced a reformation called Encomienda. This stated that natives from colonized territories must convert to Christianity, making colonization a necessity to spread religion. Also, they produced sugar, which was considered a "rich man's crop", because of the level of labor needed and the process of its cultivation.
Well Spain saw America as a chance to become richer.
I am searching for the answer too. Some possible reasons ive found are they wanted new resources that they couldnt get in Spain, or money.