I suppose one of the strongest reasons to adopt the elements of the Weird Western setting is the way it deals with friends and parties of adventurers.
A sheriff can call together a group of people to aid in law enforcement. Presumably, this little legal (and cultural) practice -- along with the concept of closeness to the people you ride with -- doesn't strain credulity when a party needs to be formed for various expeditions.
On the frontier, it's not like there loads of people. Sometimes you gotta settle for that shifty, but surefooted guy who's Billy's friend or Sally's brother -- because there ain't no other people skilled enough to bring. And sometimes you build up a professional closeness to people you don't like, because you ride with them.
And it's not just the PCs either. Other groups of people -- perhaps people who all served under a leader during a war -- would ultimately gravitate together to carve out their own lives in the shadow of the growing power of the 'nation' along the frontier.
It's not just in the 6th Gun. It was a key motif in the movie Tombstone between Doc Holliday (Val Kilmer) and Wyatt Earp (Kurt Russell).
Friendship is a reason to risk your life, to question what you believe in, and -- yes -- it's definitely a reason for revenge.
And while we're on the subject: here's to true friends everywhere.