When you think about it, they share the same roots, Castille/Spain have/had as substantially bigger army and population. Why was there no attempt of the Spanish gaining more power and annexing Portugal?
Well, there was a period of 60 years were Spain dis control Portugal. You might want to check out the answers to this question- Why Spain has never invaded/kept Portugal under its reign? which has some good links to answers in the faqs.
There was, but I can't say that I know enough about it, so I will try to point you in the right direction for reading.
It was called the Iberian Union, and it was a personal union lasting from 1580, when Portugal had their war of succession, until 1640, when the Portuguese revolted.