Levels & Effects of Gratitude
Gratitude seems to be the doorway to happiness (at least from my own experience and some research), and to receptivity and compassion as well.
Levels
As with most (all?) human qualities, it comes dualistic and transdual forms...
[Dualistic]
Dualistic gratitude is the one we are most familiar with. We are grateful for whatever matches the preferences of our human self, and not so grateful for the rest.
One of the tasks of our human self is to differentiate, and that is essential for the survival and effectiveness of our human self in the world. But when this is taken further and we take the preferences of our human self as gospel truth, and add value judgment, then we create problems for ourselves - and one form this takes is differentiated gratitude.
We now are at odds with the world. Some manifestations of the world are OK and we allow ourselves to experience gratitude for them. And many others are not approved of (as if we, tiny ants on the surface of a tiny planet, had anything to say about it), and we do not allow it to give rise to gratitude.
[Transdual]
More transdual gratitude takes at least two forms.
One is gratitude for whatever is happening. It is all God expressing, manifesting and exploring itself. It is all food for our maturing. And if we take it that way, it opens up for gratitude. Whatever happens, I can repeat the mantra of "thank you, thank you, thank you" and open up for appreciation and gratitude. It all is experiences, raw materials for my development and maturing. It is all gifts. Some come in a form that fits the likes of my human self, and others don't. Still, I can find gratitude for it all.
Another is gratitude for the happiness and good fortune of others. Whenever I see or hear about someone with good fortune, I can rejoice in their happiness. It could be anything from good health, good looks, intelligence, compassion, being deeply human, having wealth, having food on the table and a roof over the head, acting in a way that benefits others, insights, clarity, friends, family, being alive, and so on. There is no limits for what we can find to rejoice in, for any one individual and among all individuals. And as rejoicing turns what could be resentment and jealousy into happiness (and helps us from a more dualistic to a more transdual view), there is no limit to what is fuel for our happiness.
Effects, evolution & differentiation
There are many effects of gratitude. It helps us open up, find a new receptivity. It helps us connect with ourselves, the larger world and others. It helps us find a sense of connection with and compassion for others. And it brings happiness.
Happiness is conditioned on particular circumstances, and in particular - it seems - gratitude.
[Evolution]
From an evolutionary perspective, it seems that there is a simple set of relationships that connects ratitude and happiness, and that it all has survival value. Gratitude tends to come up when situations match the preferences of our human self. And these preferences again tends to have survival value. When gratitude comes up, as a signal that our situation (most likely) is favorable to our survival, there is also happiness. There is a release from worrying. And this makes us want to experience it again. There is a sequence of favorable circumstances > liking of these circumstances > gratitude > happiness, which then makes us seek those favorable circumstances again. In this case, there is of course a strong link between circumstances and gratitude, they have not yet been differentiated.
[Differentiation]
When we come from a more clear discernment, we can differentiate gratitude from the particular circumstances.
We can discern and act in a way that is beneficial for our human self and others without being (too) blinded by habitual likes and dislikes. And we can find gratitude independent of the particulars of the situation.
This is a win-win situation.
There used to be a struggle and the roller-coaster of gratitude and lack of gratitude, happiness and sorrow. Our happiness was at the mercy of circumstances. It was either win-win or a loose-loose situation.
Now, we find a way to be effective in the world and make choices from discernment, while finding reason for happiness in anything that happens. It is a win-win in any situation.