Bloodline was certainly important - a marriage of families could well end one house's power over a kingdom.

Also, consider the laws of succession differed greatly between countries. In England, a female could inherit the throne - in France, a female could not. Thus a female could become Queen of England but her male cousin might be the one who inherits the Kingdom of France.

Keep in mind that the modern nation state is a much more recent idea, and that we are talking about titles being inherited more than entire empires. Charles V was Emperor of the Holy Roman Empire as well as King of Spain (and the colonial Spanish Empire) - but he voluntarily abdicated in favor of his younger brother who became Emperor of the Holy Roman Empire and his son who became King of Spain. The idea that they could become a unified superpower simply didn't exist given the disparity/difference in how the two entities were run or simply existed.

Ultimately though, I don't think it's all that different from why modern countries don't get together and just unite. Even within the EU, there is major dissent between the various countries about the level of integration. It could be pride, an issue of practicality, economic/social differences, etc.

The Habsburg family actually created an empire - mostly by means of marriages. They could have united Europe, but the foreign policy of Richelieu stopped them during the thirty year war.

My question though is aimed at isolating the reasons why kingdoms such as France and England didn't simply arrange a marriage to unify their empires, or when they did, this union didn't continue on to the next generation to bring in others such as Spain, Austria, and Denmark.

This was basically what the Hundreds Year War was about, the English house of Plantagenet had a better claim on the French throne than the French House of Valois did. The reason why there was a war was because the French nobility refuse to accept Edward III's claim to the throne and instead nominated the Valois branch of house of Capet onto the throne. One of the reasons is that there's too many regional interest groups against the an incoming foreign ruler in a large percentage of the cases. For something as powerful as a throne there were simply too many other claimants to it for one house to hold it for a long time. Even in England itself, the Plantagenets would destroy themselves fighting for the throne.

There was also the fact that even personal unions didn't have to be unifying, the Hapsburgs under Charles V for instance, theoritically ruled Portugal, Castille, Aragon, and Austria and the Netherlands at one point in time. Yet the governments of each of those places remained independent and not integrated with each other. This is especially striking of Portugal and Spain, even though they are right next to each other and had the same king, they were not integrated with each and Portugal would eventually gain independence. The means by which pre-modern monarchs could rule over vast territories was simply too limited for a pan-European monarch to be viable on the long run.

That is a good question, actually. An important aspect of this dynamic is the fact that each kingship or lordship is a distinct legal entity. James I of England, for example, was also James VI of Scotland but the two remained separate realms who happened to have the same sovereign. It wasn't until the 1707 Act of Union that they became a single political entity. So even where a single person had amassed a huge stretch of territory, they often were not a single cohesive unit which makes them subject to instability. It is difficult to be a king in absentia and hold on to your rule, but with a large empire it is impossible to be present everywhere, especially in an era in which travel was so difficult and lengthy.

It is also important to consider the sentiments of the leading lords within a country as well. A King can't rule effectively if he spends all of his time fighting his own lords. And the lords' aren't going to be pleased at the idea of a stranger staking a claim to the throne. Jealousies and grudges between countries are a powerful deterrent to this kind of thing.

That is part of the reason why Mary I of England, for example, forced her husband Phillip II of Spain to sign a pre-nup saying that he could not claim title to the English throne. The English leading citizens were not about to allow a foreign born Catholic of all people to come usurp the English throne just by marrying Mary.

Basically long story short, administering and ruling a country was hard work. Ruling an empire was even harder. Ruling an empire that spanned the entirety of Europe - with all the cultural, religious, ethnic and linguistic diversity therein - is nigh on impossible. Other countries will see the weakness inherent in trying to control all that and use the opportunity to snap up your stuff.