A character who has quit smoking or drinking keeps one, last unsmoked cigarette, or one last bottle to remind him of why he quit. May be a dare, a test where he can prove to himself that he has quit for good, a quitting device to prevent him from buying more, or a memento to remind him of what they were like before quitting.
Sometimes, if the character goes through a particular level of stress, [...] [tvtropes.org]