Why did he get involved in another's matters again, he wondered. He had just so kindly been given a free meal and now was about to cause trouble in the old man's shop. The kid being beat down on was painful to watch for reasons more than the physical abuse being dealt. It was certainly non-proportional to the crime but the offender was also a child. But Jin knew the real reason that and the following situations had compelled him to stay on.
He remembered what it was like, pouring out the discontent, the disagreements, wanting to pour out everything but somehow unable to fully express the betrayal. So instead he had turned to action. But where he had had training of the elite level, this kid had none and surely would have failed. Watching Sosuke cry out his unacceptance before that whom he had respected most tore deeply at him. He was in the school again, before his master once more, and the foundation for all the morals his teacher had taught him crumbled as the lie had manifested and ended in that duel and he had closed himself off from the world.
He knew that boy's tears well and he wanted to redeem himself, to help Sosuke where none had helped him. And in exchange, the boy would cry the tears he never could. It was equivalent trade.
From episode 3.