Disclaimer: J.K. Rowling owns these characters; I do not.
Number Twelve, Grimmauld Place has always frightened him. But without Sirius, it's more than frightening: it's terrifying. The house is well-kept, but there is an air of malevolence about it, seeping through the walls. Regulus hates the house. He loves the Black inheritance, but he hates the house.
There are a few cobwebs that will soon overrun the place after no Black is left to run it. Dust has begun to gather on a few old books. It will soon be wiped off, but it will come anew, as is its nature. Regulus sees the premonitions for the house's eventual decay, and he doesn't regret it.
He looks outside the window, his elbows resting on the sill. He knows that this house will always terrify him, because Sirius will never set foot here again. Sirius was his courage, and he's lost that, now.
There is a connection that brothers have, especially Black brothers. Cygnus and Alphard, despite their differences, always respected each other, as Regulus observed. But he hadn't realized that connections can be broken, if enough strain is applied. And tension was there from the beginning, just because of the happenings in the world. The events that happened at Hogwarts only made that tension worse.
He always knew that Sirius couldn't ever live in this house, let alone be the head of it. Sirius didn't have the heart for it; he was wild like Bella, hardly ever content to be still. He was the extrovert to Regulus's introvert. And in the bottom of his heart, Regulus always knew that Sirius would one day be gone. The bonds that held him to the Blacks never left any visible welts on Sirius; he slipped out of the ropes with ease.
And he wished he could be like Sirius, sometimes. He wished that he was unfettered and unbound. But he was the brother that would always peer back during mischievous schemes to see if Walburga was there, watching with a stony and forbidding glare while Sirius would only look ahead. Sirius was, in an odd way, more of a parental figure than Orion and Walburga were; despite his irresponsibility and reckless disregard for rules, he cared about his brother in a way his parents never did. He taught him spells that he had learned at Hogwarts, even though it was against the law; he was the shoulder Regulus would cry on when he was a child afraid of the dark.
He stops himself from thinking about comforting things like this. Sirius had left; his parents, this Black family, is all he has.
This house, dark and imposing as it may be, is all he has.
