Second half is still one of my favourite conversations between the characters in DoW2, specifically the:“But do they respect you? Do they even know you?”
The idea that despite the bonds of brotherhood between Astartes, it might not be uncommon for them to be....well, lonely.
Plenty of them, especially veterans like Cyrus, who served in the Deathwatch, probably have secrets or at least sensitive information to keep, had unique experiences or suffered specific trauma. Maybe even have thought/opinions, which should not be voiced for one reason or another. Lost those within the chapter closest to them. Any number of things really.
Anything that might isolate them from others and these things build up over the decades or even centuries of service. Until they are decorated members of their chapter, respected or infamous, but not really close to anyone anymore.