There isn't really an explanation for this, other than I was stuck for a subject in English class and I thought that Shintaro got over his father's death pretty quickly. I do not own InuYasha and co. Please read, review, and enjoy.
Shintaro ran towards the desecrated shrine, feet kicking up flower petals as he went. He was in such a hurry to make sure that Hakushin-shounin was safe that he forgot to watch the ground he ran across. He tripped over something covered by the flowers, and land on his face. Scrambling to his feet, he looked for what tripped him.
Hidden from view by the flowers and the leaves was the skeleton of a human hand.
That… That robe… Shintaro thought, panicked.
"It's dad..! Dad!" he cried, his voice a mixture of fear and grief. He felt tears well up in his eyes. Just as quickly, he wiped them away.
Once, his father had told asked him "Shintaro, when I die, will you shed tears?"
Shintaro had been young then, and he had not truly understood what 'death' meant, and had answered "No, dad."
"Why not?" his father had asked.
"Because," Shintaro smiled, "I want to make sure that my older sisters are protected, so I will not cry until I know that they are safe."
"Good," his father replied, kneeling in front of his son and putting his hand on his shoulders. "I want you to be strong, and I want you to protect your sisters when I'm gone, okay?"
"Okay!" Shintaro had replied with a smile.
Remembering those words now, Shintaro swallowed his sobs for another time. Now was the time to find out what or who had killed his father, and protect his sisters and the people that had come to help them. Grief would come later, for now, all he could do was help and protect those in need. Like his father had told him, he would be strong.