How and When to Be Happy

Gemara: "I praised the happiness" (Kohelet 8:15) refers to simcha(happiness) related to mitzva, and "What is happiness there for?" (ibid. 2:2) refers to simcha that is not related to mitzva.

Ein Ayah: There are two opposite mistakes that are made regarding how to properly deal with a person’s natural inclinations. A person is born with an untamed nature, which has to be developed. However, this should not be done by trying to totally negate his nature, for this will not succeed. Rather, he should use wisdom to determine the purposes of his characteristics and integrate them in a manner that serves the purposes for which they were created. Therefore, it is a mistake to despise a characteristic and try to eradicate it.
On the other hand, it is also a mistake to accept one’s characteristics and build a lifestyle around them as they are. This is an approach of evil that leads to death. Rather, one should avoid the lowly path that can lead from the characteristics and use them to carry out intellectual/spiritual goals, along the lines of divine and moral norms.
Happiness is a normal characteristic of healthy people. However, such happiness, when unbridled, can be connected to matters so lowly and absurd as to melt in the face of the intellect. The happiness that is connected to sensual desires will not really satisfy a person and will lead to disappointment when he discovers their emptiness.
However, these observations should not bring people to reject the gift of happiness. Rather, we should take this healthy tool and fill it with truthful applications. We should internalize the idea that happiness was made for these proper goals, not to try to satisfy one’s wild fantasies. Thus, the gemara advises to apply Kohelet’s questioning of the value of happiness to happiness that is not related to mitzvot, where over-exuberance is childish and spiritually damaging. What is positive issimcha of mitzva, which includes happiness about having grasped truth about Hashem and His Torah, the joy of having performed acts of goodness and kindness, and performing all of the obligations of the Torah and the intellect, in action and in thought with happiness. When one does so, he will uncover the great potential of his characteristic of happiness and establish it according to its essential nature.
This is what the pasuk means by "praising the happiness," referring to the specific and natural happiness that applies to a person in the depth of his spirit, after he has elevated himself with wisdom, piety, and ethics. Indeed, simcha of mitzva, i.e., happiness connected to honest intellect, is praiseworthy. In contrast, when happiness remains in its low level, it will not last over time, as life and intellect will "slap it in the face" and say, "What is happiness there for?" Childish reveling is based on falseness and nothingness. It only appears, from a distance, to be real happiness, but it is revealed by investigation to be false. Rather, happiness exists in order to be connected to mitzvot. "I will rejoice in Hashem; I will have joy in the G-d of my salvation" (Chabakuk 3:18). About the type of happiness that celebrates intellectual attainment and following the proper lifestyle and mode of behavior, it is said, "The heart of those who seek Hashem will be happy" (Tehillim 105:3).