Opal's father is trying to teach Opal a life lesson, to help her become more tolerant, and to see things from other people's perspective when he makes her apologize to Stevie Dewberry. By making her tell Stevie that she is sorry if she said anything that hurt his feelings, the preacher is guiding Opal to take responsibility for her own actions even if she feels she has been wronged and does not understand why people act the way they do (Chapter 18) .
Stevie Dewberry inarguably torments Opal. He whispers about her with his brother when Opal passes by, and he says mean things about her and terrible things about her friend Gloria Dump, whom he calls a witch. Opal quite naturally fights back, yelling right back at Stevie and his brother. Sometimes, Opal gets so "wore out" from hollering at the Dewberrys everyday, that by the time she gets past their house, she feels "like a soldier who had been fighting a hard battle" (Chapter 13).
The preacher finally talks to Opal about Stevie because Mrs. Dewberry has told him that her son claims that Opal "called him a bald-headed baby". Opal admits that she did, but defends herself, telling her father all the things Stevie does to provoke her. The preacher listens, but still insists that Opal take the initiative and apologize. He wants her to be the one to take the high road in her behavior, and he tells her that even though Stevie may have "a strange way of going about (it)", he probably just wants to be her friend (Chapter 18).