How does Ethan learn for certain that Mattie loves him?

Ethan learns for certain that Mattie loves him when she shows vulnerability and openness about her feelings, particularly during their intimate moments together. Their shared experiences, such as their time spent alone at the farmhouse and the emotional connection they forge, further confirm her affection. Additionally, her distress over the prospect of separation amplifies the depth of her feelings for him. This culmination of emotional exchanges leaves Ethan convinced of Mattie's love.