Walls
Mortarion hates Sanguinius. He never shows it and never tells anyone, but every time he sees the Angel, he wants set those stupid wings of his on fire. It doesn't help that he is quite aware just how irrational his feelings are. Sanguinius never did anything to warrant such enmity from his brother.
When he looks at Sanguinius, he sees perfection. Unlike Fulgrim, he does not chase after it and it is not what he begrudges. It's the ease with which it comes to the Angel. Despite growing up on an irradiated death world, Sanguinius always looks beautiful. He can be splattered with gore and tired from days of fighting, but he will remain graceful and awe-inspiring. Mortarion knows that he looks like he's recovering from a death-threatening illness on his best days.
Mortarion is not a people person. He doesn't like small talk and he tends towards pessimism. People are afraid to strike up a conversation with him. Sanguinius has no such problems. He can merely enter a room and suddenly everybody will adore him. He doesn't even have to bother.
But most of all, he hates Sanguinius, because whenever they are in the same place Sanguinius tries to be nice. He should be able to see that they won't ever be real brothers and yet he offers companionship. It's like he can't see there is and always will be a wall separating them.
"We'll go this way and flank them," Sanguinius explains, drawing a line on the map with his gauntleted finger. "You have to keep them occupied."
Mortarion looks at the map, his face shadowed by the hood of his grey cloak. He has said roughly three sentences during the whole planning session—Sanguinius has been counting. The Angel feels like he is talking to a wall. Why won't Mortarion respond?
Just as Sanguinius gets ready to add something, his brother nods. The Angel waits, but only silence follows.
It's probably the most awkward situation he has ever found himself in. Sanguinius simply cannot think of what to say to his silent brother. So, he tries what has worked so far and places his hand on Mortarion's shoulder.
"You can tell me if there's something you don't like," he says, smiling encouragingly at his brother.
It fails. Mortarion shrugs his hand off and shakes his head.
"It's a good plan," he says quietly, but somehow it makes Sanguinius feel like a door has been shut in front of his nose. Mortarion stands right next to him, but he could as well be miles away.
Sanguinius doesn't know Mortarion. Whenever he tries to strike up a conversation, his brother remains silent. When he has talked with him for the first time, it felt like he was talking with air; he could not tell what his brother felt or thought.
He does not like Mortarion; his brother won't let him. He shuts him off and hides in himself, using silence as fortress. He does not dislike Mortation: he does not know him well enough to feel anything like that.
If anything, he feels mildly frustrated with him. They have a background similar enough to share at least a bit of understanding and yet it is not the case. Sanguinius would like to know why, but he is not dying of curiosity.
In the end, he treats Mortarion the same way he would like his brothers to treat him, simply because he hopes that one day it will pay off.