Trust. When you think about it (or mistype it like I just did), it reminds you of the word "truest." Generally, players do want characters that are true, as it's a virtue. Having a player that's trusting is considered a drawback. Earning trust is a sign of power; giving it is a sign of weakness.
And yet, players want to develop relationships with one another. Steel, Sand, and Sunsets has two characters who are brothers. It's great that they didn't grow up together, but—and I might be inferring here—they should be more like brothers by the time session six rolls around.
A note on our campaigns. Because we usually have several capable storytellers, we tend to rotate through games every two months. For example, Terry can run Werewolf for two months, Josh can run D&D for two months, etc., etc. That's eight weekends, but of course it's usually less than that because life happens (even in the middle of the life happening, yet more life can happen). I aim for six episode arcs, personally.
So these two players need to be brothers. Like all relationships, it's based on shared experiences and trust. Trust that others will make right decisions and trust that they will aid you. Characters approach one another with the baseline level of suspicion they hold towards strangers. That has to be replaced by learning about the other player, watching them do the right thing and reward trust that's given to them.
That's the difficulty here though; players always make good on one another's trust. Because the party is going to be together for a while, they'll have to deal with the long-term consequences of failing to gain someone's trust. It can even break up the group (in-character), or worse for me as the storyteller to contrive a reason for them to stay together. If the trust is do-or-die, I can lose a party member.
Especially running a group of guys, pride is a factor too. Sometimes it's hard for one character to change their behavior to benefit another. Other times, there's an amicable trade arranged that only requires minimal trust; the characters become business partners with a good work history. A beginning to be sure, but not really what I (or my players) are looking for.
What I'd like to do is make a game where players have multiple characters starting off. They play all of those characters minimally and as characters work together and develop (and survive), a team with connections naturally survives.
Evolution is an important principle. I'd love to see a game utilize evolution as a central principle. Not the advancement and increasing complexity principle, but the paradigm of what's best suited to survive, survives. Using the development of natural, volatile personal bonds to make a party would be a good application of those principles.
Maybe that'll be my next game: Guess Who: Party 'til You Drop.