Hiya!
I went with "Compatible personalities" and "Other".
We are an older group (the youngster in the group USED to be 36?...'ish...?; but my nephew is up for the next couple months so he's temporarily the youngest at 19...or 18...'ish...

). As a bunch of old farts we have families, jobs and a shared "childhood/highschool baseline". If someone makes a joke about the Barbarian "Wanting a new drug...or maybe duck", we all "get" the song references. If someone says "That's a big twinkie..." after hearing the description of an extraordinarily large wagon or mundane item...we all get the reference. Oh, and none of them have their phones out once the session gets going.
As for "Other"...it's usually related to, well, being older. We enjoy playing games with each other. Hanging out, rolling dice, making up stories, etc. As such we generally don't sweat the small stuff. This kinda falls into the "Agreeing how to play the game system", but it's more than that. It's how we actually
approach the game system. For instance, when we play Hackmaster 4th, we generally stick to the rules if there are any for the situation before making something up. With 5th, it's 50/50 on that; sometimes we will delve into the rules, others we just pick a DC and make a roll. With a game like
SUPERS! Revised Edition it's usually 'make it up on the fly' because 9/10 times doing that is what the rules call for anyway.
Also falling under "Other" is the simple fact that where I live, there are not a huge amount of players that don't already have game groups. It's the capital city of the Yukon (Whitehorse), but our population is only about 30,000. Hell, the population of the entire Territory is only about 36k, and the Yukon Territory is 186k square miles (482k square kilometers). Or about 20k square miles LARGER than California (or about 2/3 the size of Texas?). Needless to say...our player base is fairly consistent and groups tend to stick together for years or decades; not a lot of cross pollination, so to speak.
^_^
Paul L. Ming