While "were sullen" implies a feeling of gloominess or resentment, there are a number of antonyms that portray a more positive mood. For instance, the opposite of sullen could be joyous, cheerful, or lighthearted. It could also be optimistic, hopeful, or optimistic. Other antonyms for sullen could include friendly, welcoming, open, and approachable. Those who are not sullen are usually talkative, lively, or extroverted, often eager to share their thoughts and feelings with others. Regardless of which antonym is used, it is clear that sullenness is not the only way to express or react to one's emotions.