That is a deep question.
I think true happiness is the ability to love what you're doing, in the present moment.
Even something as routine as doing the dishes can a happy time, if you savor the experience.
The feel of the hot, soapy water.
The sense of accomplishment in getting something done.
The gratitude for dishes to do and a warm house to do them in.
To me, it's all about being present in what you're doing.
Especially if I break a plateau. Solving difficult problems makes me happy. Gaining a deeper understanding of any of the sciences makes me happy. Jokes make me happy. Watching whose line is it anyway makes me happy. Bothering my friends and seeing their exasperated expressions makes me happy.