Happiness is not a final and stable state of being, but moments in time, like every other emotion.
We can find ourselves comparing our current emotional state to our ideal, often discovering reality is falling short of our expectations.
When we define happiness as something to work towards, we have shaped its boundaries, the rules which apply to our version of happy and can feel pressure or disappointment when the conditions are not met. It can make us more unhappy.
Acceptance that happiness is not a constant state and doesn't require certain things to happen first releases us from feelings of failure. By letting go of ideals and searching for happiness as if it were a commodity we are opening our minds and hearts to the present moment and allowing our emotions to emerge as they wish without judgment or expectation.
It is rather difficult to describe what makes a person happy as it is subjective and occurs when our emotions are awake and our senses alive. We just are happy at the time, and probably so caught up in the emotion that we are absorbed in the experience and not thinking about it.
Most of our strongest memories whether good or bad, have an emotional attachment to them and we know this when we reflect back and find the smile we had back then returns to our face, and indeed tears can have the same effect. We relive the emotions not the thinking behind it.