Why are the Scandinavian countries always ranked high on the happiness scale?

Those counties have homogenous populations.  They have similar work ethics, pastimes, tastes, and general good character.  They do not have to address the social disorder generated by cultural minorities that cause strife, crime, pollution, and put a heavy burden on the social welfare state. 

Because there aren't any blacks in those countries. Blacks make people unhappy because blacks are negative and commit crimes.

They are very safe democratic monocultures.  Other similarly ranked countries include New Zealand, Austria, and Luxembourg, also very homogeneous.  The lowest ranked countries are the ones in which there are clashes between Christians and Muslims, Muslims and Hindus, Muslims and Buddhists, or have 16 different official languages, or are one-party authoritarian systems.

They're nations rife with natural resources, including oil, and they have such teeny tiny populations that they can provide some solid basics to their inhabitants. That said, counterintuitively these nations also have relatively high suicide rates. 

Atheism. They are the countries with the most atheists. They are not as judgemental as christian countries. Therefore the people are happier.

Population is too less 

Socialdemocracy

They have a healthy multiparty-democracy, and a lot of citizens are smart enough to vote for parties to the left of the USA-Democrats and to join a trade-union, rather than believe that hunger and homelessness make people productive.