The classic one is the social judgment angle, which feels evergreen. Think about the whispers at family gatherings, the disapproving looks from his peers who think he's having a midlife crisis, the awkwardness with her friends who see him as an authority figure rather than a boyfriend. It's not just external, though. Internally, he might wrestle with timeline anxiety—fearing he won't be around for her later chapters, or that he's holding her back from a more age-appropriate life. I'm always more drawn to when his past becomes a third wheel in the relationship, like an ex-wife or grown children who resent the new dynamic. That adds a layer of domestic tension you don't get with younger couples.
Sometimes the obstacle is less about society and more about power, especially if he's her boss or mentor. The fear of exploitation, real or perceived, can poison even genuine affection. He might overcompensate by being overly cautious, which she reads as coldness or lack of commitment. What I find most compelling is when the age gap itself isn't the main problem, but it amplifies other issues—different cultural references, energy levels, or life priorities. That feels more real than a story that just makes everyone cartoonishly prejudiced.