Cutting.
It provided such a rush. The flow of blood that trickled down his arm calmed him. Proved he had control over some part of his life, and he loved it.
Each cut meant something to him. This large gash was when Sasuke left. Eight more scars for close encounters, and four more for failing to retrieve Sasuke from Orochimaru.
Three scars because Sakura couldn't keep her mouth shut, and insisted on acting like an ass. Two more scars because Kakashi honestly didn't care for him. Not one bit. An additional two scars because Iruka remained clueless by choice, shrugging it off, like nothing.
One scar because of idiotic dreams that, in privet, he thought he couldn't accomplish and four more because of the villagers that still tormented him, tearing his very soul apart with just their eyes. Very skilled indeed.
But none occurred because of one person. That person, no matter what, couldn't hate him. While there were jokes passed around, it wasn't intended to hurt him, just rile him up. No scars occurred due to Sai, whose kisses eased his pain and healed his scars.
For Sai, he would stop cutting, and scaring his arms. For Sai, he would do anything. A feeling he had felt toward many others, but never as strong as this.
For Sai, there would be no more blood.