What motives led to Iberian overseas expansion?

As ever, I would say that the motives behind expansion are inevitably pecuniary. South America was unpopulated by other Western civilisation, and it would prove far easier to conquer such nations so far away compared to their attempted conquest of civilised countries, like Britain and France. They expanded into many other regions of the world, including having other colonies in Europe, but what caused them to do this would be mainly to gain money (for the King, of course) and to boost the national status and obtain a substantial empire.