The Buddha tells us that we should not attach to the worldly things of this life because all will eventually pass. The path to enlightenment winds through many lifetimes. The teachings of Buddha look past our current temporal life.
Happiness is a slippery concept. You can define it and know that you feel it, but that feeling can change radically in an instant. To feel fulfilled in life, a person must be engaged. Obviously this can take many forms. Without the process of engagement, there is nothing to be filled with. Haidt is teaching that engagement is the path toward happiness. Granted, but then you must ask… Engage in what?
Haidt says ‘personal life, work and something larger than ourselves’. And Buddhism says that everything is larger than ourselves. If you push personal and work without the larger world in mind you may be engaged and you may feel happy, but those things that carry on from your life, those things that are larger than you, will scarcely be affected. If you follow personal and work disjointedly from the ‘larger world’ a chasm will grow, which you are bound to face at some time.
There is nothing antithetical to Buddhism about immersing in life with gusto. And there is nothing contradictory about entertaining bemused detachment to the outcome of your activities. We continually see how change (e.g. storms, floods, waves, wars… ) can destroy plans and happiness. And these things can occur at any moment. So the wise person, the sage of Buddhism, delights in the experience of life, yet realizes its inherently impermanent and transitory nature. With that in mind, she walks directly into the spiritual. And it is through the spiritual that we grow as humanity.
Haidt has nothing to say to correct the Buddha. The mixture of personal and spiritual in your life is an individual matter. Finding happiness is not necessarily spiritual and finding spirit is not necessarily happiness.