The New Economics Foundation researches every year the level of the happiness in the world.
To calculate the European Happy Planet Index, the foundation  ranks countries for their carbon footprint, life expectancy and life satisfaction.

In this year ranking Iceland comes top of the European HPI with 72% index. Sweden (2nd place) and Norway joins Iceland at the top of the HPI table. The score for countries ranked in the end – Estonia, Bulgaria and Luxemburg – is lower than 30%.

North European countries like Denmark, Iceland, Finland and Sweden do best in terms of life satisfaction.
The Scandinavian nations have some of the lowest per capita carbon footprints in Europe, despite also being amongst the richest and happiest nations. Some of the differences can be explained by access to domestically available renewable energy sources, but not all.
Scandinavian countries are the most efficient - achieving the highest levels of well-being in Europe at relatively low environmental cost.

According to Stefan Olafsson, sociologist of University of Reykjavik, Iceland victory is easy to explain. In Iceland, like anywhere else  people are happy when they have friends and family. Most of Icelanders are married and Iceland is 2nd country in Europe in terms of  birth-rate. But that’s not the only reason. Iceland is a rich country with almost not existing unemployment, never in conflict with other countries and never occupied in its history.

Poland was ranked before Hungary and Czech Republic on the place nr 19 with the score of 43%.

Source: „Rzeczpospolita” 19.07.2007
and:
http://www.neweconomics.org/gen/european_happy_planet_index_160707.aspx