How simple and frugal a thing is happiness: a glass of wine, a roast chestnut, a wretched little brazier, the sound of the sea. … All that is required to feel that here and now is happiness is a simple, frugal heart.”

- Nikos Kazantzakis in “Zorba the Greek” (1946).

However, on the other hand

A happy life consists not in the absence, but in the mastery of hardships.”

- Helen Kellerin "The Simplest Way to be Happy" (1933).

We are happy when we decide that we are happy. Happiness is a subjective state of mind. The most important thing in being happy just might be able to stop and see the situations where one can be happy. Instead of wanting more and better things, our happiness may well be founded on the things that we already have.
On the other hand, happiness is very much about enjoying doing something that makes one happy. Humans are active animals and just enjoying good meals, and the sound of the sea just might not be enough for most of us. We need activity that can make us happy. We need challenges, and we need hardships that we can overcome. However, the road to happiness can be spoiled by consequences of eating and drinking too much, or it can be ruined because of trying to achieve too much in too short time.

Striking the right balance with enjoying the small joys of life and at the same time striving to achieve something worthwhile and even larger than life just might be a possible road to happiness. The Epicurean ideal of moderateness in all we do could just be the real key.
There just might not be a single or simple solution for achieving happiness, but achieving a state of happiness just might be a precarious balancing act, where different and even conflicting needs are taken into consideration. As I love to say; “Too much of any good thing just might be bad.”
On the other hand, there just cannot be any kind of objective criteria for happiness. This is true, even if there can be objective criteria for material or even mental well-being. These things can also be measured and quantified. However, happiness is something that we just feel in our bones, and nobody other can reliably ever know if we are happy or not.

Santorini - Wikipedia

One can well develop criteria for measuring happiness, but they are doomed to be just subjective attempts of faking objectivity. So strongly subjective thing as personal feeling of happiness is a thing that only a person him- or herself can ever asses reliably.
Even asking if a person is happy is not a reliable way of determining true state of happiness at all, as there are countless reasons why people would lie of their true mental state. They might well exist a need to appear stronger and more successful than people really are. Unhappy people may be classed as losers by the society and one can feel the need to lie about his or her true state of happiness when asked about it.
In addition, the relationship between the person questioned and the person doing the questioning can be a decisive factor. One may well feel the need to lie to please the person doing the asking.

In the end, dollars or euros have precariously little to do with real feeling of happiness. After the basic needs are satisfied, it is so much more about how we personally relate to things that we already have. Of course, a very basic requirement for happiness is a safe society and one which divides the fruits of the economy as justly as possible. However, after they have been reached, people need to realize what they already have.
Unfortunately, the human mind seems to be built in a way where things are extremely easily taken for granted, and their presence is realized only after they have been lost. I am naturally speaking of the post-industrial western nations here; in all too many countries striving for happiness still consists of striving to survive.

Wisdom

The foolish man seeks happiness in the distance;
The wise grows it under his feet.

James Oppenheim in "The Wise"