The biggest correlation of happiness that I know of is sensory clarity. Both purely as a correlation in that people who make finer distinctions report higher baseline levels of well being, and as a causal intervention in that making fine distinctions improves both positive and negative experiences.
This is self evident from experiments with things like mindful eating, but may be less intuitive for aversive stimuli. Making fine distinctions in pain seems to make it worse initially, but then makes it much much better and this technique is starting to become much more widespread for people with chronic pain
https://www.shinzen.org/wp-content/uploads/2016/12/art_painprocessingalg.pdf
This hypothesis stands in contrast to some common frames:
Common frame: happiness is a reward for virtuous behavior. (Reinforced by parenting styles)
Common frame: happiness is selfish
Common frame: happiness is a future state
Instead, I like viewing unhappiness as a set of reflexive behaviors that encourage the coping strategy of numbness. This is a short term bandaid with long term big downsides.
How do you account for unhappiness borne of empathy for the suffering of others?
Investigation of this state with sensory clarity will likely reveal it as a part that hasn't yet learned to make the sympathy/empathy distinction and thinks that feeling others' pain a la sympathy is a load-bearing motivational structure.
Is there a reason that I specify sensory clarity?
For a start, it is less fuzzy a concept than mental clarity as well as more directly trainable (you can literally just choose to have more clarity about physical sensations right now) seems to affect mental clarity with training.
But also, sensory clarity and baseline "being in the present moment" go hand in hand (probably a positive feedback loop). The fewer mental filters (often with negative affect) that are being interposed between the raw signal input and what eventually enters consciousness the more sensory clarity there will be. M
any people report enhanced color saturation and other sensory "upgrades" as they become more "enlightened".