Abu Hurairah (May Allah be pleased with him) said: The Messenger of Allah (PBUH) said, "Two matters are signs of disbelief on the part of those who indulge in them: Defaming and speaking evil of a person's lineage, and wailing over the dead.''
[Muslim].

Commentary: Both the sins mentioned in this Hadith are such that if a Muslim thinks them lawful and still commits them he will become a Kafir. To defame someone's lineage means to disgrace or humiliate somebody by saying to him or to her: "Your father belongs to such and such profession'' or "Your mother is such and such / so-and-so'' or "You are a weaver, blacksmith, laundrer, cobbler, etc.''

Mourning and wailing means expressing qualities of a deceased person through weeping, crying and lamenting loudly. Imam An-Nawawi said that such qualities are attributes and acts of the disbelievers and practices of the pre-Islamic period, or Jahiliyyah.

[Note: Feeling sad when someone dies is obviously human feeling that you can't stop, but what is forbidden is excess crying and wailing, which a person can control if they chose to]