Happiness depends on ourselves. It’s a central purpose of human life and a goal in itself. It has side of the perfection of human nature. Since man is a rational animal, human happiness depends on the exercise of his reason. But in the end nobody can say for sure what Happiness really means. Because each time it’s completely different, even for one person it can change among his life. 

You don’t need to understand why a woman on the painting is happy, and is it even permanent. You can translate your own definition thanks to her.