Guilt
Disclaimer: Bleach is not mine.
Ichigo had a secret.
He was guilt-ridden for the death of his mother. That much was no secret to his close friends and remaining family. The fact that he missed his mother horribly was another unspoken fact.
And they were right--he did feel guilty for killing his own mother, for causing the death of the mother he had adored.
But … they were also wrong.
What they assumed--what Tatsuki and Rukia and Orihime and Chad and Isshin and Yuzu and Karin all assumed--was that his chief source of guilt was from the actual death of his mother. It was a logical conclusion--he had killed his own mother, though his father had reassured him that it was not his fault. Anyone would feel guilty for that. And Ichigo did.
The real guilt from the event was for the living, not the dead.
Yuzu and Karin had been so young when it had happened. They were too young at the time to remember more than vague impressions of their mother now that they were older. He, the older brother that loved them, had taken that mother away.
Isshin might be crazy, and the giant posters of Masaki everywhere were creepy. But he had loved his wife. Now he could only have the memories of her. Ichigo, his son and firstborn, had done that.
So he protected his family. And really, even though he liked to think he would be a good enough person to protect them all anyway, there was no way to know that. He had not had a choice, when it came down to protecting them.
He had a debt to pay to the family he had hurt. A debt that he would pay until he was reincarnated as someone else and could not remember his former life. Maybe even then--some things transcended even death, as Ichigo had discovered.
In the end, his every action from the death of his mother onward came down to that endless debt and the weight of his own crushing guilt.