Why did Spain?

Answer 1

For the same reasons other European kingdoms did...to increase revenue, land holdings, prestige. Also, for Spain, "converting" more people (even at the point of a sword or gun) to Catholic Christianity would make Spain look better in the eyes of the Pope and therefore the Roman Catholic Church.

As most of Europe was Catholic in the early 1500s, this was a very big deal. Also keep in mind Spain as a unified country really didn't exist until 1492, when the Spanish forces kicked the Moors out of Spain forever. At this point Spain was a relatively weak and poor country, compared to the Holy Roman Empire or France.

So, how to get rich, big, and powerful? Colonize, by any means necessary. That Mexico is so corrupt and relatively poor is a legacy of the brutality of Spanish colonialism and oppression.

Answer 2

There are a great many reasons why Spain does what it does. This country desperately wants control over certain resources.

Answer 3

to find gold and spices from India and China.