The answer is not complex.
Happiness is simply a state of inner freedom.
With a bit of self-insight, every individual can answer that question for himself. It is freedom from the secret angers and anxieties we tell no one about. It is freedom from fear of being unappreciated and ignored, from muddled thinking that drives us to compulsive actions, and later, to regrets. It is freedom from painful cravings that deceive us into thinking that our attainment of this person or of that circumstance will make everything right.