Light-heartedness is a term used to describe a carefree and happy-go-lucky attitude, often associated with humor and an absence of worries. However, there are several antonyms that indicate opposite qualities to light-heartedness. For instance, seriousness refers to the quality of being earnest, grave, and solemn. Melancholy indicates a state of sadness, gloominess, and depression. Similarly, sternness refers to an imposing and strict attitude devoid of humor or warmth. Finally, preoccupation suggests distraction and a state of being too busy or worried to enjoy life fully. These antonyms highlight the contrasting emotions and attitudes that are the opposite of light-heartedness.