More Life 2017 Headcanons (Spoilers)
Crews on the International Space Station are generally 3 people but can support up to 6 or 7.
David who set a record for consecutive days in space was clearly up there the longest.
His crewmates on his first expedition were probably two Russian cosmonauts who went back down when their expedition was over but David asked to stay up there. Maybe this happened for a couple of expeditions prior to canon and Mission Control let him simply because it's harder to find and replace a flight surgeon/medical officer and they assumed that as a doctor he's monitoring his own health.
Then those two crewmates were replaced by Kat and Sho (both seem closest to David and both showing more signs of fatigue than their crewmates so they might have been up there the longest apart from David--Kat's strained smiles, suggests she's possibly cracking under pressure (or just fed up from dealing with Rory) & Sho who desperately wants to go home to his wife and newborn daughter. Maybe they were sent up with a third US astronaut meant to finally relieve David but Person Number 3 got sick (illnesses of all kinds are made worse in space due to a number of factors that have nothing to due with aliens--isolation, stress, disrupted sleep, fluid shift of microgravity, recycled air, radiation, etc...) and had to go back down to Earth again.
David is perfectly happy about this turn of events (because he truly never wants to go back down to earth) and cheerfully volunteers to stay on.
David is fond of Kat and Sho, they are his friends. He's closest to them. He's fond of reading around them--for him that counts as socializing.
Kat is very protective of David during the press/public relations tour.
In the deleted scenes David is the one Sho confides in about his family, his traumas and anxieties.
And during the events of the film David risks his life trying to rescue Kat--and lies to Sho about being completely ready and suited up for an emergency EVA, knowing full well Sho wouldn't depressurize the airlock for him otherwise.
Then the latest additions to the crew would have been Hugh (xenobiologist to study the alien), Miranda (quarantine officer to contain the alien), and Rory (newbie who's already homesick and misses his dog, seemingly harmless clown... & also possibly Deadpool--given the other fuckery of not disclosing all the quarantine measures and trying to kill the crew it would make perfect sense to also have an assassin onboard intended for containment in a worst case scenario. That would make all three of them there specifically to deal with/contain the potential Martian organism.
That might also be another factor of Miranda falling for Rory despite their evident clash of personalities--they have a shared secret.
Normally the last three up to the station would be relieving the last expedition but these are special circumstances retrieving the Mars sample requires all hands on deck so they have a full crew of six.
Also the deleted scenes make it clear that once the Mars samples are aboard they aren't letting anyone come back down for fear of contamination. Though at this point they're more thinking alien cooties than alien invasion.
The movie begins with David trying to remind Rory about how to move in microgravity. Maybe Kat put him in charge of helping the newbies get their spacelegs. David also sits by Kat and across from Rory during meal times. You couldn't have more opposite personalities then David and Rory--Rory being a chatterbox and constantly flaunting the rules and David preferring to sit and read quietly and carefully follows protocol. Maybe its assigned seating--with Kat trying to pair up David with "the problem child" Rory in the hopes David would be good influence and get Rory to better follow the rules. But canon suggests the reverse might have occurred--Rory had more of an influence on David. Because David followed the rules (which led to Rory's death) he begins to more often shirk the rules (more like Rory) to try to save the others...
