Affection stems from the Latin affectionem ("disposition, relation, state of body or mind"), originating from afficere ("to act on, affect, or produce a change"), a cobination of ad- ("to") and facere ("to do, make".  Entering English around the 12th-13th century via Old French afection.  Originally, affection referred to a general disposition or state of mind (including "passions" opposed to reason) before narrowing to specifically mean liking, fondness, or love.  Affection shares its roots with affect and is distinct from affectation (which comes from the related Latin affectare, "to strive after" or "pretend"). 

According to the Oxford English Dictionary (OED) , the noun affection was first used in English during the Middle English period (1150-1500), with evidence dating back to the 14th century (specifically around 1340) to describe an emotion, state of mind, or inclination. It derived from Anglo-Norman and Middle French afeccion, stemming from Latin affectiōnem.  The earliest recorded usage is found in the Ayenbite of Inwit (1340) in the sense of a state of mind or inclination.  The noun affection appears frequently in late 14th-century texts, such as John Trevisa’s translation of De Proprietatibus Rerum (c. 1398).  While the 14th century marks its introduction, the term is well-documented throughout the Middle English period (1150–1500) and later.  

Affection in storytelling—encompassing love, tenderness, trust, and intimacy—is a vital, often understated, element that elevates a narrative beyond mere plot points.  It functions as the emotional heartbeat of a story, enabling readers to invest deeply in the characters and their journeys.  Whether between lovers, friends, or family members, scenes of affection provide necessary "breathing room" in high-stakes stories, make characters more relatable, and deepen the emotional resonance of the narrative. 

Drives Emotional investment (The "Heart" of the Story): 
Creates Connection: Readers are more likely to remember how a story made them feel rather than specific plot details. Affection creates a palpable emotional connection that makes readers care about the outcome. 
"No Tears in the Writer...": If the writer does not feel emotion (including affection) while writing, the reader will not feel it while reading. 
Provides Relatability: Showing tenderness makes characters, even in extraordinary circumstances, seem human and accessible. 

Strengthens Character Development: 
Reveals Vulnerability: Affectionate moments reveal a character's softer side, showing their capacity for care and vulnerability, which makes them three-dimensional. 
Showcases Growth: A character who learns to open up and express affection, particularly after a period of coldness or trauma, offers a powerful character arc. 
Defines Relationships: Affection (or lack thereof) is a key tool in showing the intimacy, tension, and trust between characters, establishing the foundation of their relationship. 

Enhances Plot and Tension: 
Raises the Stakes: When characters genuinely care for one another, the reader fears for their loss. Putting a loved one in danger creates higher stakes than a mere plot-driven threat. 
Catalyst for Action: Love and affection are powerful motivators. Characters will take risks, make sacrifices, and do "interesting things" (both good and bad) to protect or win the object of their affection. 
Creates Contrast: In thriller, mystery, or war genres, small, quiet moments of affection provide necessary contrast to the surrounding conflict, making the tension more effective. 

Improves Narrative Flow: 
Provides "Squee" Moments: These moments—like stolen glances, tender touches, or unexpected gifts—give the reader a "warm fuzzy" feeling and a break from intense, fast-paced action. 
Builds Trust: Scenes showing affection (e.g., caring for someone's safety, listening to them) build trust between characters, which in turn builds trust with the reader. 

Types of Affection in Storytelling: 
Romantic: Passionate, tender, or playful love, which serves to advance plot and character. 
Platonic/Friendship: Deep bonds between friends, which can be just as strong and crucial to the plot as romance. 
Familial: Parental, sibling, or extended family affection, which can drive intense protective motivations. 

Affectionate, well-developed relationships are the foundation of compelling fiction, helping characters to evolve and transforming a story from a simple sequence of events into a memorable, emotional experience. 

Just the facts:

AFFECTION Definition & Meaning - Merriam-Webster

AFFECTION Synonyms: 215 Similar and Opposite Words | Merriam-Webster Thesaurus

AFFECTIONATE Definition & Meaning - Merriam-Webster

Affection vs. Affectation: What's the Difference? | Merriam-Webster