Aaron had told his little Cupcake that he will marry her after her college, right at twenty-one. He was already late, so why wouldn't he agree, especially after her sweet, open proposal when she could order him to do so?
His heart indeed melted for her efforts to make him feel special and let him know that he is the prime person in her life. Although he knew everything about her feelings before, it felt more pleasing and precious to hear it from her.
He also knew even after putting everything into words, she won't be satisfied. She would still feel those words lack to tell her real feelings. And he would love to fill those absences and make her content.