Byakuya's parents died when he was young. He used to be such a happy, spoilt and carefree child. Actually, let's rewind a bit.
He used to be such a happy, bratty, disciplined, but, happy child. Being the next head of the most noble household cannot be ever carefree or necessarily spoilt. That was when his parents were here anyway.
Ginrei-jisama was nice, wayy nicer than any of the other elders, who never thought Byakuya was good enough. They never thought Soujun was good enough either though, so Byakuya didn't let that get to him.
His parents' deaths was his first warning.
He should've known.
Byakuya met Yoruichi a little while after.
With Yoruichi came Kisuke, and with them came the Shiba siblings. He and Kaien immediately hit it off, cussing at each other and throwing projectiles while the other three just sighed, but amused and accepted their fate. They were this little squad of five, all had burdens normal people wouldn't understand even if they tried, because after all, you can't just understand the lifestyle of the heirs of the first four noble houses.
Byakuya was happy with his little makeshift family(he will never admit that thought).
Then Yoruichi and Kisuke left. Without a goodbye or anything, leaving the three in the wind. Byakuya deemed them dead, but Kaien was hopeful as ever, saying that they will come home someday, back to them, and they'll be perfect again. Kuukaku was a a bit more realistic, but still hopeful. Byakuya let them be. They can hope for him.
Things were well for a while. Kaien and Miyako get married, and Byakuya was the groomsman. He was immensely happy that he was, but of course he didn't show it, and all was well. Soon after, Byakuya met Hisana.
She was a beautiful and kind woman, worthy enough to make him throw away his heritage, mobility, his life, everything. But las, she was just a Rukongai rat and he the head of the greatest noble house, such liaison is forbidden.
Kaien was the one who convinced his little "brother" to chase for his happiness, while Miyako secretly taught Hisana the rules of a noble house. They still needed the elders to approve, and if they didn't, he really would have to throw away everything for the one he loves. He was going to too, but with the help of Kaien, new head of the Shiba house, it worked.
As I told you, everything was good for a while, you know, until Hisana died of an illness that should've been easy to cure. He found Rukia a year later, and asked(begged) Kaien to take her in, for as long as he is the captain of the sixth squad, she will never fit in. Rukia never did know about the relationship between her beloved Kaien-dono and her cold brother, because if she did, she will think that Kaien devoted all this time to her was not for her, but for her brother.
("Just so you know, I'm not doing this for you. It's for that sweet young lady you're talking about."
"Never.")
So when Kaien died, killed by the same sword in the same hands of the girl he saw everyday, Byakuya didn't blame Rukia, for it was Kaien's stupidity and pride who got him killed.
He was always an idiot.
The next morning, Byakuya went to the Shiba mansion. The place was chaotic, because the two heirs were gone. As Byakuya stood in front of the mansion, he learned his lesson.
"Kuchiki-sama! I'm sorry for the confusion, but do you have any idea where Kuukaku and Ganju-sama's whereabouts are? Please, anything will do."
"I'm afraid not." He said as he turned and walked away, not waiting for the servant's reply, then he stopped briefly to turn his head bit to say this last thing to the servant,
"You should stop looking for them. Kuukaku never liked nobility."
Then Rukia was sentenced to death, and Byakuya couldn't save her, for he was weighed by the promises he made to dead loved ones. He saw Yoruichi again, and he was outraged. Why did you leave us? Why? For it was her fault that everyone left him eventually, just like how Rukia and Renji will. So Byakuya fault her with fake fire in his eyes, because he could not truly hurt someone who loved dearly. He killed Renji because Renji tried to do the right thing. But he let him live. Byakuya was only human after all, no matter what the elders told him, that he had to be above human, he was only human, so he let Renji live.
When he saw Kurosaki Ichigo, he thought, He can save her, so he let him live, too. So when he finally had a taste of defeat for the first time in a century, he knew that this was the right thing to do.
When he saw Ichimaru Gin's Shinsou reach towards his beloved little sister, Byakuya thought, I can do the right thing, this time, and so he did.
The Kuchiki men were cursed, after all. They could never follow their loved ones. That's what he thought on his bed at the hospice.
Maybe I can be happy again?