A person who is only truly happy when he/she is upset. The inability to experience any or all of the following: happiness, laughter, joy, comradery. A tendency toward feeling bitter, scorn, spite, wrath, vengeful. See also killjoy and buzzkill.
A Perpetual Malcontent is one who complains about the quality of free food. A perpetual malcontent is critical of an event everyone else enjoyed.
Someone who is not content. Usually applied to someone who has either lost or is not happy with their lot. Malcontents tend to bitch and moan a lot about their current situation.