The OECD measures life satisfaction across a number of countries. One of their findings is a link between education and satisfaction, people with more education are more satisfied. You can download the OECD data and test this for yourself. The graph below shows life satisfaction plotted against years spent in education, there is some indication of a positive correlation but this correlation is only 0.38 (measured using Spearman test).
There is a lot of scatter. If instead we look at earnings and satisfaction, the correlation coefficient is 0.74.
The correlation between earnings and education is also stronger at 0.41 than between satisfaction and education.
So perhaps the apparent correlation between Life Satisfaction and Education is actually due to a stronger correlation between wealth and satisfaction, the link being the more education you have the more likely you are to earn more. Maybe we are more materialistic than we like to admit.