Happiness is an internal process. Happiness comes from inside, you are the cause of your happiness. You can make yourself happy or upset, it's all up to you. The way someone feels about certain things is was causes them to be happy or unhappy. The way someone interprets certain things is also a factor of their happiness, the way the feels inside about things is what's going to cause their happiness or unhappiness.
        

To begin with, you are in control of your happiness, you can make yourself happy or unhappy. In the book Ethan Frome, Mattie is the cause of Ethan's happiness, and the way she makes him feel inside is what makes him happy. On page 55, it says;" His wonder and his laughter ran together like spring rolls in a thaw." This shows that Mattie made Ethan feel so good and happy inside that his wonder and laughter were genuinely real. You produce your own happiness or unhappiness. Ethan creates his own happiness when he's with Mattie because she's very similar to him and he really likes her. Mattie makes Ethan feel good inside, unlike his wife Zeena. Ethan creates his own unhappiness inside when he's with Zeena because he doesn't really love her, but he chose to marry her, also Zeena is in the way of Ethan and Mattie being together.
       

Pursuing this further, happiness just like any other emotional experience, is produced internally. In connection to Ethan, his happiness and love towards Mattie come from the inside, when he sees her his heart starts beating fast and he gets all giddy and happy. This happens to Ethan because Mattie gives Ethan true happiness and she makes him feel really happy which makes him genuinely care about her. In the book, on page (164), it says;" For the life of her smile, the warmth of her voice..". This shows that Ethan gets so happy with Mattie that he just adores everything about her, it's like he wouldn't know true happiness without Mattie. She taught him what real happiness is, because that's what he feels when around her, real happiness inside. This is why happiness is measured with questionnaires and scales, you'd miss important information if you measured it only as a physical feeling.
        

Yet another reason why happiness or unhappiness is an internal process all depends on how you interpret any situation. Ethan uses that short time he gets with Mattie when he picks her up to have fun and spend it with her. He uses that short time to get closer to her and enjoy it, and spend all that time in happiness that she gives him. For that short amount of time, he forgets about his worries and problems and he lets himself feel happiness and enjoy it. He makes the best out of this situation, he interprets it as a time to be happy and enjoyed by him and Mattie. From the book Ethan Frome it says that;" He had been afraid that she would hate the hard life, the cold and loneliness; but not a sign of discontent escaped her. This shows that Mattie took a bad situation, which was moving to this place with a hard life, she brought life and happiness into Ethan's world, she made his life better despite the cold and lonely winters. She interpreted the situation by finding a way to bring warmth into the winter.
            

Ethan has a hard life, but with Mattie he looks at his life differently which brings him true happiness and makes him feel genuinely happy. Being with Mattie, and looking at every bad situation in a good way brings Ethan real happiness inside and makes him feel genuine happiness. Everything about Mattie makes Ethan feel genuine and internal happiness. His real happiness is what helps Ethan figure out that he really loves Mattie and he can't imagine life without her. The happiness she brought and continues to bring him, really changed his life in a way that he tries to make the best out of everything. Zeena brings Ethan unhappiness because he never really loved her, she never makes Ethan feel the way that Mattie can make him feel. Everyone can feel happiness or unhappiness externally, but it's still always produced internally and then it could cause it to be external too, or just  stay as an internal feeling.