You will never find the word happiness in Zen literature. It is a Western dream even included in the US Constitution.
It is a positive emotion and, like any positive or negative emotions it comes and goes.
I identify 2 types of happiness based on their origins external or internal.
Coming from the outside world, external happiness is almost infinite as far its source is concerned. A new car, new job, new house, jack pop, vacation, new sex, food, traveling, addiction, more of this, of that, etc. We are the objects of our own source-generated emotion.
Since external happiness is always a transient emotion:
1) Attachment to it may trigger suffering when the source is phasing off.
2) We are always looking for new sources to make us happy again or happier when the previous source is fading out as it always does.
So, we are then chasing a mirage and it becomes an ongoing source of frustration, disappointment, and suffering.
On the other hand, internal non-material “happiness” comes from self. It appears from self-discovery and we nurture it from practice.
How to transform superficial & transient happiness into something deeper and durable?
External happiness is almost impossible to achieve continuously.
Another important point to remember: as said before, external happiness is, by definition dependant on its external agents whereas the internal one is only dependent on yourself: discovery, practice, and control.
The solution is to discover, learn and apply equanimity. It is a state of mind.
It is not a thought nor emotion; you are creating a new state of mind.
Zen describes the word equanimity as: “Neither a thought nor an emotion but rather a steady conscious experience of reality as it is, including acceptance, resilience to its negative content”
Equanimity brings serenity and contentment.
Zen practice and teaching thru mindfulness meditation should transcend external and transient emotional happiness in order to switch and practice equanimity, serenity and contentment.
Serenity, contentment via equanimity can/ is achieved during awakening when:
1) We realize the sources of suffering such as persistent attachment, ongoing desires, hatred and not being able to differentiate genuine reality from mind-made fiction.
2) That we have almost not controlled life, events, people, and our own thoughts and emotions.
3) That things, events, and people are what they are and not what you want them to be.
4) That our thoughts are just that and do not represent the truth even coming from our own mind.
5) That past and future are virtual even if you watch pictures of the past and plan for the future.
6) That we are alive only in the present moment, not yesterday and not tomorrow.
7) That we are practicing mindfulness on the go including bringing your mind where your body is and what its is doing rather than daydreaming in another space-time.
8) When we are able to observe and accept negativity rather then fighting it.
This is again equanimity.