We sometimes take for granted the hereditary or divine right of kings, handed down to us from (we suppose) ancient times. The fact of the matter is that such rights were invented by certain kings along the way for their own purposes. The Germanic tribes of Anglo-Saxon Britain would have none of that.
If you were a king in these times, your job was to hold on to your throne as long as you could. This almost always meant fighting to keep it. Whether you were fighting your own people or people from other tribes or territories was open to question.
Anyway, you as the king got that throne, most likely, by winning an important battle or by just plain claiming the throne, assuming that you had enough good warriors around you to back you up. But let a few of those warriors get followers of their own, and watch out! And if you think that by having a son you can solidify your legacy, forget it. If your son proved to be a weakling and no match for your bloodthirsty brother, who had been just waiting for you to die in order to launch a campaign of his own, then your son had better watch out!
It seems odd to us now to consider such an idea, since we know that the Britons before them and the Normans after them pretty much believed in a hereditary succession on the throne. And beginning the Middle Ages, British and English history are full of examples of kings claiming that their right rule came from the very heavens.
But, there it was. The king of a Germanic tribe or territory was just the biggest, baddest boy on the block, with the most toys and friends at his disposal.
Of course, this all began to change as Christianity slowly took hold, and it certainly changed when the Danes invaded in later centuries. But the former is a story that has already been told, and the latter is a story yet to come.