The antonyms for the word "are sullen" are plentiful. Some of the opposite words that come to mind include cheerful, happy, bright, lively, and enthusiastic. When someone is sullen, they are typically moody, gloomy, and uncommunicative. Conversely, a cheerful person will be optimistic, friendly, and approachable. A happy person will exude positivity, contentment, and joy. A bright person is sharp, alert, and intelligent. A lively person is full of energy, animated, and vibrant. Finally, an enthusiastic person is passionate, excited, and eager. All of these antonyms serve to highlight the stark difference between being sullen and embodying a positive outlook on life.