Why does Pa feel ashamed that Noah is such an "awkward" child?

From the text:

"Pa thought he knew why Noah was strange, but Pa was ashamed, and never told. For on the night when Noah was born, Pa, frightened at the spreading thighs, alone in the house, and horrified at the screaming wretch his wife had become, went mad with apprehension. Using his hands, his strong fingers for forceps, he had pulled and twisted the baby. The midwife, arriving late, had found the baby's head pulled out of shape, its neck stretched, its body warped; and she had pushed the head back and molded the body with her hands. But Pa always remembered, and was ashamed. And he was kinder to Noah than to the others. In Noah's broad face, eyes too far apart, and long fragile jaw, Pa thought he saw the twisted, warped skull of the baby. Noah could do all that was required of him, could read and write, could work and figure, but he didn't seem to care; there was a listlessness in him toward things people wanted and needed. He lived in a strange silent house and looked out of it through calm eyes. He was a stranger to all the world, but he was not lonely."