Pain
Remus has had plenty of experience with pain.
He's broken bones and needed stitches and has pulled muscles. He's chewed on his own flesh and has probably left a mountain's worth of bloody bandages behind him over the years. Pain is comforting, and it follows him around like a little lost puppy that always finds its home in him. Remus thinks that he wouldn't go as far as to say that he goes out looking for pain, but he certainly doesn't reject it when it comes sneaking up on him.
He thinks that he's had lot of practice with pain. He's had plenty of opportunity to feel it out and measure his worth against it. He knows how much he can take without breaking, and he's aware of how it can affect him. He's better versed in the subject of pain than he is in any of the classes he took in school or studied on his own.
And maybe that's why it comes as such a surprise that this hurts so much.
Sirius is dead. And the naked emotion that he wants to express is sitting there openly on young Harry's face. And while Remus has been jealous of many things and many people, he never quite thought he'd be jealous of a teenager for being able to let go in ways that Remus has long since forgotten how to.
He tries to trace it back, hoping to find the kid in him who can let this out. But he's done this for so long that it's hard to remember. Shutting out. Shutting down. Pushing away. Pushing back. Distance, distance, distance.
He doesn't want to be a part of the action. He doesn't want to participate. And he doesn't want to play well with others if it means letting others in. Because, to be honest, he's done that and he's tried that, and that was a pain that could break. He doesn't want to be involved in the emotional upheavals of death and destruction and adolescence.
He understands the need, and he sympathizes. Really, he does.
But, at a point, it's just too much to ask. Remus will lend his knowledge. He'll lend his skills, his diplomacy, his contacts. He'll lay down his life if that should be necessary.
His memories, his thoughts, his feelings, his pain? Those are non negotiable. His blood, his sweat, and his body; he'll gladly extend to the cause, to the Order, to the people who for some reason still call him friend. His soul is not up for grabs, however. It is not on the bargaining table, and Remus will walk away should anyone force his hand. It is his to horde and to hide away from the world if he should so choose, and he chooses.
It's taken years to deaden himself to this degree. It's taken a decade to distance himself with the winsome sort of deception that he has. He's numbed. He's spent an entire lifetime inhuman. And he's finally succeeded in feeling as alien on the inside as people view him on the outside.
So, Remus deals with death the best way he knows how. He steals himself, and presents a good face to the world. When Harry rails at the universe, Dumbledore, Voldemort and him; Remus offers distant platitudes and hopes for the best from afar.
Because Remus has had plenty of experience with pain and no matter how well versed in the subject he might be, he only knows one way to deal with painful times.
Alone.