Being fully involved in your daily life activities, relationships and work, spending some time planning pleasurable, engaging and meaningful activities, helping, forgiving, being grateful where we can and using our personal strengths in the service of humankind are the key to happiness
The following information will throw some light on science behind happiness.
Grosberg, Merlin, and Zak of Claremont Graduate University created a biological model of happiness that reveals levels of hormones and the quality of relationship a person has explained 50% of the deviation in satisfaction of life. The primary focal point of the model is the hormone and neurotransmitter oxytocin.
Oxytocin (OT) promotes attachment to others and is correlated with sanguine social behaviors. The brain’s dispense of OT is linked with the personal encounter of empathy.
Alterations in progesterone and estradiol during a woman’s cycle affect the receptors for OT. Reports from women are consistent with this effect—an increased satisfaction with life during the late follicular phase (before ovulation when estrogen is high) and a decrease in happiness during the luteal (premenstrual phase).
The biological model of happiness shows the release of OT affects mood directly, particularly happiness. OT potentiates the delivery of the neurotransmitters serotonin and dopamine. The release of the OT reinforces social interactions as positive situations by decreasing anxiety (serotonin) and sensation (dopamine).
Specific structures in the brain possess a neural involvement in coordinating a response to reinforcement. A reward is usually used as a reinforcer—delivery of the reward increases the probability of a behavior and satisfaction. Primary reward examples include survival needs, such as water, food, and sexual contact. Secondary rewards, such as money, derive value from the primary rewards.
When rewards are delivered there is a release of dopamine. Happiness, is considered the response of dopamine on the brain. The stimulants for dopamine differ in each individual.
"Living is like being chained at the bottom of a shallow pond with my eyes open and no air. I can see distorted images of happiness and light, even hear muffled laughter, but everything is out of my reach as I lie in suffocating agony. If death is the opposite of living, then I hope death is like floating."
"That is the simple secret of happiness. Whatever you are doing, don't let past move your mind; don't let future disturb you. Because the past is no more, and the future is not yet. To live in the memories, to live in the imagination, is to live in the non-existential. And when you are living in the non-existential, you are missing that which is existential. Naturally you will be miserable, because you will miss your whole life."
"You will never be happy if you continue to search for what happiness consists of. You will never live if you are looking for the meaning of life." -Albert Camus
I think happiness is directly proportional to the conformity of your thoughts, actions, and desires with your essential nature. For most people, there is a gap between their essential being (the version of themselves where all their potentialities are actualized) and their existential being (the version of themselves that actually exists). The further a person bridges this gap, the more they will discover themselves and the happier they will be. As a theist, I think that in the process one draws closer to God, the personal ground of one's being. In Zen Buddhism, this process is conceived as discovering one's Buddhahood, one's Buddha nature, or the Buddha within.
"Happiness, true happiness, is an inner quality. It is a state of mind. If your mind is at peace, you are happy. If your mind is at peace, but you have nothing else, you can be happy. If you have everything the world can give - pleasure, possessions, power - but lack peace of mind, you can never be happy."
You may want to look at the Grant Study from Harvard, a longitudinal research of more that 85 years that shows that joy is more important that success. https://news.harvard.edu/gazette/story/2017/04/over-nearly-80-years-harvard-study-has-been-showing-how-to-live-a-healthy-and-happy-life/
"Happiness is the consequence of personal effort. You fight for it, strive for it, insist upon it, and sometimes even travel around the world looking for it. You have to participate relentlessly in the manifestations of your own blessings. And once you have achieved a state of happiness, you must never become lax about maintaining it. You must make a mighty effort to keep swimming upward into that happiness forever, to stay afloat on top of it."
-Elizabeth Gilbert
Please also have a look at these useful ResearchGate links.
Happiness has not a universal definition that make analogous sense to everybody. Happiness stems from realizing and accepting the present moment with its full potential. The present moment is beautiful and fundamentally perfect, so complete mindful attention and enjoying bliss found in the present moment is the key to happines.