Formal/Clinical:
* Masochist: This term refers to someone who derives pleasure from pain or suffering, but it's often used in a sexual context.
* Self-saboteur: This term describes someone who consciously or unconsciously undermines their own happiness and success.
Informal:
* Grump: This is a general term for someone who is habitually bad-tempered and unhappy.
* Negative Nancy: This is a slang term for someone who is always pessimistic and complaining.
* Debbie Downer: Similar to "Negative Nancy," this term refers to someone who brings down the mood of others with their negativity.
* Misanthrope: This term describes someone who dislikes or distrusts other people, and can often lead to feeling miserable themselves.
* Woe is me: This is an idiom that describes someone who is overly dramatic and self-pitying.
Other possibilities:
* Contrarian: This term describes someone who enjoys taking the opposite view of the majority, even if it means being unhappy.
* Cynic: This term describes someone who believes that people are motivated by selfishness and that nothing good can come from anything.
Ultimately, the best word to use depends on the specific situation and the individual's motivations.