Being sullen is often characterized by a gloomy, sulky, or melancholic disposition. Antonyms for this word include upbeat, cheerful, and sunny. These are all words that connote positivity, happiness, and a bright outlook on life. Instead of being sullen, one can strive to be jovial, merry, or even exuberant. By choosing to embrace a more positive mindset, individuals can cultivate more fulfilling relationships with others and enjoy their lives to the fullest. Other antonyms for being sullen include ebullient, optimistic, and effervescent - all of which embody the opposite of a gloomy or sour demeanor.