In general I’m quite skeptical of the “positive psychology” movement, as it encourages the individualisation of some problems that are really social. For instance, one might be out of a job, improverished, be depressed, anxious, etc — essentially as a consequence of living in a capitalist/atomised/otherwise awful society, and this stuff often gives the message that the problem is not society, but yourself. You just need to be happier and practice positive thinking. That idea squashes all possible motivation to actually change the system, makes people feel terrible about themselves, but better if they do think in some positive ways that are predominantly individualistic (or based on interpersonal relationships).
In other hands, positive psychology is more immediately harmful — cancer sufferers, for instance, being told that if you don’t “think positive” you’ll get worse. Well of course no medical professional should actually say this (although the placebo effect etc is real), but I’ve certainly heard of it among those who are sick, survivors, etc. Barbara Ehrenreich has written some interesting stuff about this (see e.g. here).
Having said that, “positive psychology” often focuses on having better interpersonal relationships, building better connections with others. That is all to the good — although, to the extent it does so, it is a fairly minor palliative to the atomisation produced by capitalism, and the atomising technologies it has spawned (from television to social media, etc).
It is true of course that we all need to figure out the best way to live a happy life — just for a properly understood notion of “happy”. I tend to agree with Bertrand Russell on this — I recall him saying somewhere that the first step to happiness is, first of all, to realise that the world is terrible, terrible… terrible. Only having fully internalised that (everything from human mortality, to cosmic insignificance, to the ways capitalism and sexism and elitism/hierarchy destroy most of the things in life worth living for), can one really appreciate the tenderness of human kindness, the precarity of human life, the preciousness of human love and solidarity — what human life is and what it could be, and live a fulfilling, deeply satisfying, happy life.
I don’t think many people can fully internalise all that and come out unscathed, but I do think that any claim to happiness not grounded in these facts of the world is a superficial one. A wise and happy person is also a battered one. It’s a hardline view, but the world is still a hard place — though it should not be.
Only by the struggles of those unhappy with the world, can we make it a world which systematically produces happiness.