Why do people volunteer? Obviously it must be because they find some satisfaction in it. But they may be forced to do it (say, by peer pressure or because it improves one's CV), yet one can still argue they appreciate the volunteering because they find a benefit in it: without it, there would be adverse consequences. It thus seems unavoidable to find a positive relation between volunteering and happiness, unless one is able to tease out the circumstances of volunteering. Add to this the endogeneity issue that people may be volunteering because they want to spread their happiness, or because they enjoy good circumstances that allow them to work without pay.
Martin Binder and Andreas Freytag use the British Household Panel Survey to study whether volunteering makes happy. I am not sure their reduced-form estimates are able to capture the subtleties I mentioned in my first paragraph with propensity score matching. I find more promising their inclusion of personality traits to take care of selection bias in volunteering, although personality cannot completely be ruled as exogenous. Also, their quantiles regressions can potentially highlight some heterogeneity that can be useful for our understanding of the relationship. In the end, they find that volunteering makes people happy, no surprise here, and more so the longer they volunteer. The quantile regression, however, reveals that the happiest individuals do not derive happiness from volunteering, presumably they are happy for other reasons. The least happy ones do enjoy volunteering much more. I wonder whether this comes from some decreasing marginal utility of volunteering, and whether this ties in with the fact that the poor are more generous, as discussed before.