Mind wandering is the brain exploring the problem space. It’s where the brain goes when we are not intentionally focusing on something (or, to be precise, when those parts of the brain are not engaged in processes that are experienced as intentionally focusing on things, with the understanding that “experiencing” is also a product of the brain). Problem spaces being what they are, the human animal is not always in a cheerful mood when its mind is wandering. Not necessarily unhappy – but maybe a notch or two down on the happy-ometer. But there are also times when exploring the problem space is exhilarating, a real joy: following leads that may or may not bear fruit, discovering unknowns that had been unknown, making progress here and there (of the two-steps forward, one back sort – or vice versa). And coming to a better understanding of all the hurdles along the way. Finding patches of light that promise of further clearings within the thicket of one's mind.
Of course the brain journey will include plenty of dead ends and a tendency to keep tryingtried-and-false pathways because the process has got stuck on repeat and we can’t yet see another way out of the rut. Then it’s good to take a break from all this exploring and reconnect with the perceptual world. The brain will keep working on the problems behind the scenes even when we are smelling the roses.
Strong opinion: a 6 on the scale of 0-10 does not constitute “unhappiness”, pace that highly overrated paper, "A Wandering Mind Is an Unhappy Mind" (a title that misrepresents the actual findings). For me, 6’s are fine. Nor do I care that people are slightly happier when perceptually engaged with their environment as opposed to when their attention is following the rabbits in their mind. Yes, engaging with the world, especially in ways that give pleasure, can get us out of a funk. And “being present” has its own rewards
Happiness is a good thing. By that I mean that it's reasonable to want to occupy the 6-10 range on the happy-ometer more often than not.
It’s just that there is so much else that matters.