The Space BetweenThere are things he keeps from Sirius, certainly. But he keeps many things to himself. He would not tell some things to anyone, not for all the money in the world -- not even for love.
For example, he has never told anyone that he likes the way things seem to disappear in the darkness, how the house across the street vanishes when all the lights are out. He has never told anyone that he has a soft spot for the sort of strawberry-flavored chewwing gum one finds in Muggle corner stores. He does not talk about the fact that he often counts the number of steps as he walks down a staircase, or that he holds his breath when passing a graveyard. Those close to him may notice these things, but if they do, they never mention them.
He likes it this way. The peaceful silence that falls between himself and his loved ones is precious to him. His childhood was filled with noise, with great numbers of relatives floating through the family's house, with dull conversations in hospital corridors, with the din of Muggle London and the hum of the Underground. As a favorite relative, he was fussed over by great-aunts, worshipped by cousins, and overfed by his bald, wrinkled grandfather. As an only child, his hair was always being smoothed over by his mother, and his posture always corrected by his father. As a werewolf, there was always somebody whispering about him, always a healer telling his mother some piece of news, always a Ministry official showing up at the door for a routine check-up. He always seemed to find himself at the center of attention, despite his desperate desire to be anywhere else.
So the silence is a blessing. He seeks out people who will understand him, and is happiest when he does not feel the pressure to explain himself to anyone. He prefers not to discuss topics that are difficult or controversial if he can avoid it. In fact, he likes to avoid controversy at all costs, which, considering his werewolfism, can be quite difficult at times.
Things were not always this way. When he was younger, he got worked up over politics regularly. Some of that spirit is still there, but most of the anger he felt in those days has necessarily passed on. It does not pay to be angry all the time, when the whole world seems to be against one. At the very least, one must learn to pick and choose one's battles.
Since Sirius escaped from Azkaban, there have been different battles to choose from. He no longer has to face the fury and hurt he felt over Sirius' betrayal. He no longer has to worry where he will find his next meal, or how he will afford the taxes on his family home. Instead, he must deal with a new war that may well be more terrible than the last, with a dark and ugly house, with the ill-tempered portrait in the first-floor corridor, and with the amorphous tension that permeates every aspect of his day. Things are simpler now, but also more difficult.
It was easier to keep things to himself before Sirius returned. Then, there was nobody to inquire about the things he kept to himself. For twelve endless years, he kept nearly everything to himself simply as a fact of life. He told no one that he stole chewwing gum from a Muggle corner store on more than one occasion. He told no one that he took to biting the inside of his lower lip because he liked the pain of it. He told no one that Professor Snape has a curiously Victorian interest in knick-knacks and prefers to decorate with pointless clutter. Now Sirius is back from the dead, as he privately call it, someone is again interested in these things, these intimate details. He has grown too used to keeping secrets. He does not mind Sirius' curiosity He is glad for it, truth be told, but Sirius is forever reaching for those dark corners he has grown used to holding close.
Perhaps things will change with time. He knows he will never be as open with Sirius as his lover might like. But he has never been completely open with anyone. Sirius knows this, has always accepted it. What he is holding out for, of course, what they both know he is waiting for, is for Remus to loosen his hold on his past just enough -- just enough to share it with his lover, just enough so that his lover will feel no guilt sharing his own burden.