Whether intentional or not, every game presents an emotional tone to the player. This tone may fluctuate greatly over the course of the game, or stay mostly constant throughout. Mood Management Theory defines hedonic valence as the measurement of the emotional tone of the consumed media, and consequently its ability to alter bad moods and preserve good ones. Pleasurable experiences are high on hedonic valence, while disturbing experiences are much lower. Most commonly, games and other media used to repair noxious moods project emotionally positive tones overall, even if some parts project negative tones. Exceptions that instead primarily project emotionally negative tones, most notably the horror genre, but they often rely on a combination of absorption, mood alignment, and optimum arousal.
World of Warcraft uses cartoony graphics, high-fantasy settings, quirky characters, pandas, and funny tidbits like the Mechanostrider to project an lighthearted mood that prevails even in the games more ominous sections. These touches of levity set the frame of a positively charged experience. Players who are in an undesirable mood are likely to find the cheery aesthetics of the game more appealing than the 'hyper-realism' and muted color pallets that define many other games.
Katamari Damacy is straight up one of the joyful games to play. From the soundtrack, to the character animations, to the narrative, this game oozes happiness. Playing this game and maintaining a noxious mood seems just about impossible. A player looking for a pick-me-up is likely to put this game at the top of list of possibilities.
This is an industry standard and is empirically validated
Motivations: Mood Management
Bowman ND and Tamborini R (2012) Task demand and mood repair: the intervention potential of computer games. New Media & Society 14(8): 1339–1357.
Zillmann, D. (1988a). Mood management through communication choices. Am. Behav. Sci. 31, 327–341.