Happiness or wellbeing is a much better measure of growth and progress than, say, GNP or per capita income. What's important is that people are healthy (physically & mentally), have friends, fulfilling work, live in adequate housing in pleasant places, and (as Iris emphasised) that they possess virtues like generosity & humility. A just society will seek equal wellbeing, an objective measure to some degree, even if it must include hard-to-quantify things like personal contentment & autonomy. Of course, equal wellbeing cannot be fully realised. Some of us are like Eeyore in Winnie-the-Pooh, cheerless whatever we do, & some will suffer bad luck like illness or injury. Others are satisfied with too little because they are undemanding or accustomed to going without. Moreover, we can be wrong about what is worthwhile due to lack of imagination or education. Trying to equalise wellbeing thus requires we remedy economic disadvantages (something else Iris cares about), & help people make good choices about how to live.
Thank you for your excellent question!