The Tenth Doctor smiles, a common indication of happiness. (TV: The Next Doctor [+]Russell T Davies, Doctor Who Christmas Special 2008 (BBC One, 2008).)
Happiness was an emotion.
Individuals who were happy typically displayed it by smiling. (TV: Smile [+]Frank Cottrell-Boyce, Doctor Who series 10 (BBC One, 2017)., et al.) In some cases, happiness also induced crying, a sign that the Eleventh Doctor affectionately called "humany-wumany". (TV: The Doctor, the Widow and the Wardrobe [+]Steven Moffat, Doctor Who Christmas Special 2011 (BBC One, 2011).)
The Seventh Doctor believed that happiness was "nothing" unless it existed "side by side" with sadness. (TV: The Happiness Patrol [+]Graeme Curry, Doctor Who season 25 (BBC1, 1988).)
The Vardy were programmed to keep the colonists of the human colony planet happy at all times. When one of the colonists died of natural causes, her friends and family began to grieve. The Vardy, reading grief as the opposite of happiness, treated it like a virus and began killing the colonists as to them, eliminating sadness meant eliminating sad people. (TV: Smile [+]Frank Cottrell-Boyce, Doctor Who series 10 (BBC One, 2017).)
