"Bittersweet" traces back to the late 14th century, fusing the Old English/Germanic words bitter and sweet.  It is an oxymoron that describes a flavor combining both elements, later evolving into a metaphor for emotions—such as memory or love—that are simultaneously pleasurable and sorrowful.  Originally a noun denoting plants or drinks with a blended taste, particularly the woody nightshade or a specific type of apple.  The word transitioned into an adjective in the 1500s to qualify a taste.  By the late 16th to early 17th centuries, writers (including Shakespeare) expanded its use to describe complex psychological states where pain and pleasure coexist.  Long before English speakers combined "bitter" and "sweet," the Ancient Greek poet Sappho used a famous compound word, glykypikros (γλυκύπικρον). She used it to describe the paradoxical, irresistible nature of erotic love.  

According to the Oxford English Dictionary (OED), the word bittersweet was first used in English before 1393.  In the 14th century, the term was originally used as a noun to literally describe drinks, foods, or plants that combined bitter and sweet qualities.  The OED's evidence shows the word transitioned to an adjective in the late 1500s, and over time it took on the metaphorical meaning we use today—describing mixed emotions or experiences that are both pleasurable and sorrowful.  Iconic writer Geoffrey Chaucer was one of the first to use the term metaphorically in English literature. He utilized the phrase “a bitter sweete” to describe a mixed state of fear and hope in his work, The Canterbury Tales (specifically in "The Canon's Yeoman's Tale"). 

Bittersweet elements in storytelling elevate narratives by balancing profound loss with meaningful growth.  They prevent endings from feeling either artificially neat or overwhelmingly depressing. By forcing characters to achieve victory at a heavy personal cost, these stories mirror the complex realities of life.