How can we feel happy biologically?
Biologically happiness means equilibrium, homeostasis. Which means a perfect, mutually complementing cooperation of the comprising elements of a closed, living, natural system.
If we want to simulate this "biological happiness", homeostasis within a human environment, we would need to build such mutual relationships expressed through a state of "mutual guarantee".
Mutual guarantee is the selfless, altruistic, unconditional, mutual service in between people of a "closed" human environment, when all calculations are made for the optimal well-being of the collective above personal calculations.
Though mutual guarantee we can be both biologically and "humanly" happy.