A person must be in full control of his emotions so that he can be sad when he wants and be happy when he wants, and neither one should cross the border into the other’s territory. Instead, the sadness and happiness should stay in their places and when he so wishes, he should bring one of them out and when he wishes to change he should bring the other one out. It can sometimes happen that at exactly the same time on the one hand he needs to be happy and one the other hand he needs to be sad, and his eye should cry bitterly while at the same time his heart is rejoicing. Examples of this are on Rosh Hashanah and Yom Kippur. Sometimes the happiness should be stronger while the sadness should be light like a fleeting shadow (for example on the festivals while saying the words “And because of our sins we have been exiled from our land!”, while at other times the proportions should be reversed. Each thing should be used for its right time and how good is a thing in its right time. About this it says in Mishlei that “One who is slow to anger is better than a warrior and one who controls his emotions is better than one who conquers a city!”.
It is not enough for a person that he should make his own heart rejoice. As well as this he should also make the hearts of downtrodden and demoralized people happy whenever he can with whatever tools he has to respect, cherish, raise them up and give them pleasure, and this is considered a great Mitzvah.