It is natural for a person to feel sorrow when a relative passes away, but according to the Torah this natural sorrow is not sufficient, and one is required to mourn through halakhot and actions. This forces the person to contemplate his sorrow and consider what sin brought it about, so that he will be able to repent and live a better life.

One of the reasons for this mitzva [of mourning for a relative who has died] is something I have written many times with regard to the previous mitzvot: A person is influenced by his own actions. Since he is a physical being, he is not affected by anything abstract until he converts it from the potential to the actual. Therefore, when the punishment of the death of a relative, whom nature compels one to love, befalls him, the Torah requires him to act, which will drive him to keep his thoughts on the sorrow that has befallen him. Then he will contemplate in his soul how his sins caused this sorrow to come upon him…and this is the absolute belief of those of the precious Jewish faith. When one pays attention to this matter in the process of mourning, he will resolve to repent and will refine his deeds as much as he can. (Sefer HaĤinnukh 264)