Grief

Grief was an emotion. Grief had five stages, ending in acceptance. The Shroud fed on grief, and exploited victims who had recently lost a loved one. (PROSE: Shroud of Sorrow)

According to Graham O'Brien, "[grief] needs time". He decided to continue travelling with the Thirteenth Doctor so he wouldn't simply sit around the house, waiting for his grief regarding the death of Grace to "go away". When he did visit their home, Graham's grief manifested in his mind in the image of Grace, who spoke to him. (TV: Arachnids in the UK)

In response to negative emotions such as fear, grief and anger, humans once took medication in the future, to remove and override them. (AUDIO: Harvest of the Sycorax)

When the Emojibots first encountered grief, they considered it an enemy of happiness and thought of destroying it, they called upon the Vardy to kill anyone who wasn't happy, stripping them of skin, organs and muscle leaving behind their skeleton, which later would be used to make calcium-based fertiliser. (TV: Smile)