I always figured it dealt with the fact that at that time, Spain controlled most of the New World - namely its gold - and English ships were constantly off to the west looking to claim (if possession is 9 tenths of the law, then it's not stealing is it?) what they figured was their fair share. After all, it was the Pope who had divvied up the western hemisphere between Spain & Portugal and England was now Protestant... so the rules needed to change.

His earliest expeditions as a young seaman were not successful... but he later became a dreaded pirate ( according to Spain but not according to England). Since Elizabeth I was a major stockholder in those ventures, it's not hard to understand the difference of opinion.