How good and pleasant it would be if Klal Yisrael would be happy with whatever they are doing when it is a happiness which comes from fulfilling the commandments of Hashem. For each commandment that he fulfils he should rejoice and be happy more and more and more than one who finds a vast treasure. This applies especially to Shabbos and the festivals about which it says “Then you shall rejoice with Hashem!”, and it also says “And you shall be happy on your festival!”. One must rejoice a great happiness especially at the time of the service of praying. The holy Zohar elaborates similarly about the magnitude of the obligation to say the Grace after meals with happiness and goodness of heart to the extent that they said that it is correct that they should eat and drink things which make the heart happy just in order to be able to say the Grace after meals with happiness.
On the other hand, we all know that in those things which are dependent on emotions and not actions one needs the help of the heart in order to feel those emotions as if, for example, one tells someone to be afraid and he is simply not afraid, he will be unable to be afraid. In the same way, if a person is told to be happy or not to be happy (or any other similar emotion based feeling) he will be completely unable to make himself do anything other than that which is in his heart. However, a person is praised according to his sense and logic for he must find a way to change his heart and lead it to feeling the correct emotion through consistent holy thoughts and one who comes to purify himself will receive the help that he needs.