Okay so humans, right? We got this far because we're pack animals. We work best working and supporting each other. If someone's lacking, someone else fills in that gap and vice versa. Cool? Cool.
Now, Astartes. They're raised from prepubescence, a very important time in growth and socialization and all that good stuff. Except they're cranked up to 11, it's (often quite literally) beaten into them that the boys around them are their brothers, those guys are gonna walk into hell with you and drag you out too. They're gonna fight the literal horrors of space hell side by side with you until death do you part
Not to mention the countless serfs that tend to them. These guys got serfs for everything, they're armoured with assistance, they're washed and clothed by them, their food is prepared by them. And ships have So Many serfs. There's always Somebody around is the point
So it causes this really funny thing where when an astartes is separated from his brothers, from his serfs, from everything and everyone he knows. He seeks out some form of companionship, he needs someone or multiple someones to spend his time with. It's not *necessary*, they can live alone and handle themselves quite fine for extended periods of time.
But you see them repeatedly, they seek out companions!
In Son of the Forest, one of the Fallen sticks with the villain just out of knowing him and having a companion, even when he knows his brother's descending into Chaos. Zabriel mentions having stayed with several brothers over the years and now protects a group of humans and they clearly trust him with their lives. Another one leads a pirate band with other Fallen. Another lives on a station with again other Fallen and their serfs are noted to be close by, peeking around corners.
In Void Exile, Sharr has been entirely exiled from his Company but he's found three other exiles to travel and fight alongside.
In Outer Dark, the Ashen Claws are renegades and they eat and sit alongside their humans almost as equals. The Chapter Master even has who seems to be the 'lead' human who sits with him at the head table
In Salvation, Rius crash lands and is amnesiac when he's taken in by humans and without knowing what to do just helps the village and the people. He does the heavy lifting and dragging plows because they can't but he can.
In the Dark Angels, when they give someone penance, the DA has to wear 'pentinents robes' and they're excluded from even speaking to their others. The Carcharodons literally exile their brothers. Even in the Ultramarines, a punishment for Pasanius was to seclude him and keep him from rejoining Uriel for an extended period of time.
It's this really interesting thing where astartes are So social that their punishments are literally to be secluded and excluded. They have the innate need to be around others that they seek to fill it even under literal exile. It goes beyond just the brotherhood of War and fighting literal space hell, they're constantly surrounded by serfs or human commanders and their fellow astartes to the point they can't tolerate to be alone for long.
And it's just something that lives rent free
