Which particular part of this incident is it that you are referring to? I assume it is when Ethan talks to Mattie about her upcoming marriage. Again, as before in the novel, he says this because he actually loves her and can't bear the thought of losing her, however, what he says is interpreted in a different way, as Mattie clearly suspects that his repeated references to her forthcoming marriage and her departure from Ethan's home are because Zeena is displeased with her and wants her out. Note what Mattie says:
"It's not because you think Zeena's got anything against me, is it?"
She interprets his words that are really said out of love for her as a criticism against her character, which shows the negative aspects of verbal communication.