The leader was the person who most obviously had the qualities of leadership (I'll explain more at the end). Those qualities included:

*Good lineage. Was your father or father's father a great leader and an honorable person who paid their debts and was honest with their business? Did they come from a hospitable family that takes in travelers? Were they distinguished? Also there is a reason there is so much politics around the idea of marriage. Marriage during this period was a way of unifying families to create a stronger grouping, bring peace between the two, or a way for families to become a part of a dynasty. Since heterosexual unions came with the assumption of children, those children became the heirs to family lines that also tie to land rights. As such rulers were the ones that typically owned land or had legal precedence to land ownership (i.e. their land was taken from them and they have a "birth right" to fight for it back like King Harald Hadrada).

*Personal strength, bravery, honesty and masculinity which are all tied to personal honor. If you were not the strongest or bravest then why would I follow you into battle? Leaders including Kings were at the front lines leading the attack and showing an example to their men. Also since warband leaders were responsible for distributing the booty, if you are dishonest, how do I know I won't get my fair share? And of course masculinity which ties to strength and bravery but also ties to living in your role as a man. For instance, magic was believed to be the realm of women and while there were some men that practiced it they did so at the risk of being labeled ergi which means lacking in respectable masculine values. This could lead to your wife being allowed to divorce you as well as adversly effecting your reputation for future business opportunities. As such any remarks about ones manliness was taken seriously as it to the point of challenging them to a holmgang (a one-on-one duel) or straight up killing the person (you would be legally protected because if you didn't kill or challenge the person you were weak and thus embody the label of ergi) As such a person who was unquestionably manly and honest (which also tied to Norse masculine values) were usually candidates for leadership.

*Good hamingja (luck). Is there a sense of luck around the person or things have happened to them that shows that they have a bright future and their fate is aligned for greatness? There was a belief that luck was an entity that followed and favored certain people and it only made sense that you followed a lucky person so that they could reap from the trickle-down of luck and other wealth from that person. This is something that is hard to objectively describe but the Norse were all about talking about people's hamingja. For instance Leif Eriksson is also known as Leif the Lucky since he saved a handful of men from a ship wreck that led to news of lands further West (Vinland). Luck can also leave you and with that your support for leadership can leave you too.

Now saying the choice is obvious is very subjective to the social dynamics that went on during the time. To say a leader was always chosen a certain way is not the case because during this era in Norway, kings were killing each other left and right to claim the right to rule. What we can take out of this is that the connection between the leader and pertinent landed freemen (not all freemen had the same rights) was decided based on more personal ties (reciprocity of material and service) to that potential leader and if their peers believed the same thing too. You could become a local leader without the "royal/jarl" background and still aspire for greatness and those men were called Hersirs. Hersirs would lead a hundred (a unit of land area that is a county division) show their ability for leadership through their actions in battle which could lead to a Jarl or King bringing them into their Hird(I believe this term is primarily tied to the King of Norway) or personal retinue which can lead to more favors (jarls were replaced and replaced with loyal men), the ability to marry into the family and as such the ability to expand their powers further into the future themselves or through their kin.

The best example I have that you may be able to relate to is Aragorn in Lord of the Rings. He shows the virtues of bravery, strength and honesty that is tested in times of peril and which is known by many others around him which leads to stories that build up his reputation. He also comes from a distinguished line and royal ties that add to his credibility of being a ruler. By the end of the movie he is one of the big heroes that is instrumental in stopping Sauron from taking over mankind in Middle-Earth (no one can see or know about Frodo's ring mission as it's nature requires secrecy and thus none of the glory) that it was "rightfully" acknowledged and even pressured upon him that he was the best candidate for king Arnor and Gondor. What I mean: there wasn't an election but an organic "vote" and understanding based on the qualities of the person, that is pretty much how warband leaders were typically chosen.