Let's first define ambivalence in itself. It is a state of doubt where the individual experiences conflicting emotions due to his or her own uncertain feelings about a situation.
Main character Ethan shows this type of behavior mostly all over the novel, particularly in his actions and not so much his thoughts, or his words.
- Although Ethan is unhappy with Zeena, he is loyal to her despite of his affection for Mattie.
- Ethan is unable to stand up for himself under Zeena's calculated and cold insinuations; yet, the is very upset about them.
- Ethan is able to feel emotions for Mattie, but he leaves everything "in the air". It is not until they make that fateful, final suicide pact where he takes any action on his situation; and then, we know what happens.
Chapter 2 shows the earliest signs of Ethan's ambivalence. After Mattie's dance, Ethan walks her back home but, before that, he creeps to the side (and "creep" would be the right word, actually) to admire her from a distance. Despite his emotions, he is not willing to show his true feelings. In fact, he asks Mattie whether other boys were fancying her. This clearly shows that Ethan is not giving leeway to Mattie to pursue a romance but that, instead, he contradicts his own emotions.
These alterations of mood were the despair and joy of Ethan Frome... The fact that he had no right to show his feelings, and thus provoke the expression of hers, made him attach a fantastic importance to every change in her look and tone.
Therefore, that "alteration of mood" is what shows his ambivalence regarding his feelings.