Even as monarchs go, Charles V, Holy Roman Emperor was one lucky individual. He inherited Austria and the job of Holy Roman Emperor, as well as the Netherlands (including Belgium) through his father, whose parents were Maximilian of Austria and Marie of Burgundy (this province, but not the Netherlands, was lost to the French). On his mother's side, he inherited Spain through her parents, Ferdinand and Isabella.
When Charles V abdicated, he divided his holdings, with his brother Ferdinand inheriting Austria and the Holy Roman Empire. His son, Philip II, received Spain (which had come through Charles' mother), and the Netherlands (which had come through Charles' father).
Why did Charles V divide his Empire at all? Did the Spanish and German sides have trouble getting along? More to the point, why did he give the Netherlands, which had come through the German side of the family to Philip, along with Spain, instead of to Ferdinand, when the Holy Roman Empire was at least contiguous to the Netherlands? (Spain was not.)