Its seems to me that whilst they have a wide variety of personal situations people who are happy more than they are not and generally manage their mental health well, do have some similar characteristics.
It might be a bit cheesy and nanny state but I really like the NHS five ways to well being and I've noticed that "happy" people seem to cover all the points almost instinctively. Although, I suspect it's not original NHS work 
Five ways too wellbeing