Maniac tells Mrs. Beale that he loves her after she slaps his "trashtalking mouth" because he recognizes that her action, though harsh, is done out of love. Children appreciate discipline, and Maniac, who has been without parents or a home for so long, is no different. When Mrs. Beale disciplines him, he understands that she does it because she sees him as one of her own. This realization is overwhelming to him, and he reacts by "hugging and squeezing her and burying his face in her chest and sobbing, 'I love you...I love you'".
Maniac actually didn't mean any harm by his trashtalking. He had learned it from his friend Hands Down; "in some strange way (trashtalk) remind(s) him of church...it (has) spirit, it (has) what they (call) soul". When he unthinkingly brings his trashtalk into the Beale household, talking his trash directly to Mrs. Beale, that motherly figure is not amused. At first she is shocked, then she is angered; "she didn't like this boy bringing the vacant lot into her kitchen, and she didn't like how it fit his mouth". Mrs. Beale expects much better behavior from her children, and Maniac is no different. She lets him know in no uncertain terms that his behavior is unacceptable.
As soon as she slaps Maniac, Mrs. Beale is contrite. "Her lip start(s) to quiver", but before she can apologize, Maniac reacts by grabbing her and sobbing "I love you". Maniac had recognized immediately the love behind her action, and is overwhelmed with the sense that finally, someone cares about him enough to make him behave (Chapter 15).