Along with the obligation of mourning, the Sages taught us not to mourn too much. Even after the loss of a close relative, we are the children of the eternal God. Eternal life, which the deceased has attained, is better than anything in this world.

It is not fitting to exhibit excessive agony and grief over a deceased relative if a relative who is more distinguished and has more potential for good remains. Therefore [the Torah says] that you are the Lord’s children (Deuteronomy 14:1). [For you,] whose Father exists forever, it is not fitting to fully agonize and mourn for any deceased person…. Additionally, one should not be overly aggrieved about the harm that came to the deceased at his death. “For you are a holy people” (Deuteronomy 14:2), destined for the World to Come, where [a moment of] contentment is better that all of life in this world. (Sforno, Deuteronomy 14:1)

Further reading: For the halakhot of mourning, see A Concise Guide to Halakha, p. 105.

The soul of the deceased itself wants the remaining relatives to carry on with their lives in joy. When they fall into sadness and grief beyond that which is required, it saddens the soul of the deceased.

The will of the soul:

When a close relative passes away, in accordance with God’s will, those who remain here can no longer see him with their eyes nor hear him with their ears. The soul, however, is in the World of Truth and can see and hear, and when it sees that its relatives are grieving excessively due to its physical absence, this matter causes it pain. Conversely, when it sees that after the mourning period established by the Torah, life returns to normal and is full of positive, productive activities, it can joyfully rest in peace in its resting place.

The evil inclination tries to drag people into unnecessary sadness and pain:

One should be wary of the evil inclination, which is very cunning…the evil inclination even encourages a person to give charity for the elevation of the soul of the deceased, and to study Torah and do mitzvot in memory of the soul, as long as each such act entails sadness and pain. But as mentioned above, this is the exact opposite of the objective, which is to bring joy and gratification to the soul of the deceased. (Rabbi Menaĥem Mendel Schneerson, Torat Menaĥem, Menaĥem Tziyyon, p. 546)