Bye the way, there always is a problem in translating the title of the chiefs of Romanian countries from the past. The Romanian word is 'domnitor' or simple 'domn', which may be translated as master, but this says nothing to an English speaker. The natural translation would be king, but occidental historians kept this title for theirs, promoting instead the title of princes, which is not correctly, as princes are not chiefs of states, but their children. Why? Let's go a little in the past.
After the collapse of Western Roman Empire, the barbarian tribes, generally of German origin (Goths, Ostrogoths, Visigoths, etc.) invaded its former territory. In that vacuum of power, the military chiefs built small fortresses, from where they dominated the aria. In time, the fortresses became citadels and castles, and the successors of former military chiefs considered to be the owners of the surrounding lands. We are in full feudal epoch. Several centuries, there were not great dangers for them; nobody threatened them, as toward the west the ocean was a natural border, and as far as the eastern barbarians there was a large distance, within other people, like the Romanians, had to fight against. Of curse, small fights existed as anywhere, but they looked more as some disputes inside of the family than real wars.
The feudal lords did not feel the need of wearing the title of king. It was Pepin the Short, who had the idea of crowning himself as King of the Franks. Why? Because, in the meantime, the first real danger appeared, under the form of Arabian expansion, and his father, Charles Martel, succeeded in persuade his neighbouring lords to fight together against a common threatening, and did it in the battles of Poitiers and Tours (732). Thanks to his father's merits, the son thought he deserves to wear the title of king. Of course, nobody paid attention at him then, but some years latter, the Martel's nephew is no one else but Charlemagne, which wanted to be emperor. But this is another story.
On the one hand, we must noticed that the barbarians always wanted to remake the glory of former Roman Empire, but under their leadership, if possible. This idea persisted until recent years. (Let's not forget that some German central European countries kept the title of 'Holy Roman Empire' as far as the 19th century.) Even Odoacer, who became ruler after the dead of the last emperor in 476, declared himself to be the new Roman emperor, even if nobody recognised his as an emperor.
On the other hand, the barbarians were not pagans. On the contrary, they were Christians, even more than the Romans were. In Roman Empire, the Christianity had spread slowly, particularly among the poor people (because it is a religion of poor people), starting from the east toward west. Few people were Christians in its western part before the collapse. Besides, there was not a chief of Christian churches at that time; every bishop used to be independent. Instead, thanks to Wulfila, who translated the Bible into their language, the Goths have spread the Christianity throughout in occupied territories; it is true by sword more than by conviction. Charlemagne, shaking hands with the bishop of Roma, reached two aims: he was recognized as emperor by the church, and the bishop, as Pope and chief of occidental church. Charlemagne was illiteracy, and the empire disintegrated itself after his dead, but the idea remained. It was the church that goes on it farther, made from Charlemagne a saint, a great emperor, etc. Soon, occidental church separated itself under the name of Catholic Church, in this way the Pope becoming the single chief of the occidental church, and that who may anoint kings. As for the kings, they were considered to be of divine origin. As a matter of fact, they bring again the ancient faith, more profitable for leaders, this time under the name of Christianity, even if Jesus' doctrine was quite opposite, proclaiming the equality of every person in face of God. But the real Christian doctrine could not be pleasant for kings and a hierarchical church. Their wish of power was as great as the Eastern Roman Empire used to be still alive under the name of Byzantine Empire, but weaker and weaker, while the occidental Europe became more and more powerful.
Coming back to Romanians rulers, it is understanding now why the occidental kings did not want to share the same title with other rulers: because they wanted to keep the title of king of divine origin, and it could not belong but to few families.