"Learning to be happier" by Bruce Hood, professor of developmental psychology

That autumn term of 2018, I decided to try delivering a free lunchtime series of lectures, ‘The Science of Happiness’, based on the Yale course. Even though this pilot was not credit-bearing, more than 500 students gave up their Wednesday lunchtimes to attend. That was unusual as, in my experience, students rarely give up time or expend effort to undertake activities unless they are awarded credit or incentives. There would be 10 lectures, and everyone was requested to fill in self-report questionnaires assessing various mental health dimensions both before and after the course, to determine whether there had been any impact and, if so, how much.
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We now have five years’ worth of data and have published peer-reviewed scientific papers on evaluation of the course. ... Our most recent analysis over the longer term shows that the positive benefits we generate during the course, and the two months after, are lost within a year, returning to previous baseline scores, unless the students maintain some of the recommended activities.
... What is the mechanism underlying positive psychology?

It gets pretty broad from here. I'm choosing to only copy partial excerpts as I think it's better understood read altogether.

... Specifically, we had completed a set of studies demonstrating that, when children are instructed to talk about themselves, they thought about their own possessions differently and became less willing to share with others. Emphasising their self had made these children more selfish. This got me thinking about the role of self-focus in happiness.
... Many PPIs such as sharing, acts of kindness, gratitude letters or volunteering are clearly directed towards enriching the lives of others, but how can we explain the benefits of solitary practices where the self seems to be the focus of attention? The explanation lies with the self-representation circuitry in the brain known as the default mode network (DMN).