In this question I brought a quote that I saw in Rabbi Twersky's The Enemy Within (Shaar Press: 2002) in his chapter entitled "Finding Joy in Life" (p.104):
Rabbi Samson Raphael Hirsch points out that the Hebrew word same'ach (= happy), is closely related to the word tzome'ach (= growing). This is because human happiness is contingent on spiritual growth. If we fail to feel the joy of mitzvos, it is because we are unaware that mitzvos make us grow.
Does anyone know the source for this Rav Shimshon Raphael Hirsch?