Happiness is relative and subjective, and, it's just the title. What they've actually measured is nation's prosperity and factored in wealthfare-state social programs.
Sweden has a school voucher system, low corporate tax rate, and they have a public health care, system but its decentralized so most of the management is at the city level (like city hospitals in the US -- which do get support from state govt's, btw). Swedes also have to buy all their liquor through a government monopoly, Systembolaget.
Ireland, on the list, has a ban on abortion, except in cases where a mother's life is at risk.
Yet the accompanying article wants to claim all these countries are pretty liberal. Most of what Americans know about European gov't are stereotypes and cliches.
By the way the most common thing about the countries on the list is most of them are either monarchies or are part of a monarchial commonwealth.