The survey looks at many things. Do people trust each other? Are children safe walking to school? Do families have enough to eat? Is there clean air and water? Can people go to a doctor without worrying about money? Finland tends to score really well on these everyday things.
Finnish schools are also famous around the world. Children don't start school until they are seven. They get lots of breaks during the day to play outside, even when it is snowing. There is very little homework. Teachers are highly trained and given a lot of trust. Many countries now visit Finland to learn how it does education.
Finland has the lowest rates of corruption in the world. That means people trust their government, their police, and each other. If you drop your wallet in Helsinki, there is a good chance someone will run after you to give it back. Trust is one of the things that makes life feel calmer.
Finns are also very practical about happiness. There is a Finnish word 'sisu' (say 'see-soo') which means a quiet, stubborn kind of strength - keeping going through long winters and hard things, without making a fuss. Many Finns say sisu is more important than happiness. It just helps that happiness often comes along as well.