Political--monarchs believed power was connected to wealth and it was necessary to have an empire to gain more wealth. Spain wanted to be powerful and therefore gaining access to lands aided them in gaining political power in Europe.
Economic--as said above, power was connected to wealth. Spain had the goal to find gold and silver to increase the size of their money chest. The colonies of Spain produced huge amounts of wealth for the country. This again contributed to their power.
Religious--with the Reformation, the Catholic Church took a hit losing people and with it money and power. Spain being a Catholic country, they were tied to the Pope and relied on the power of the Church. Getting colonies allowed the Spanish to convert people to Catholicism.