It's a surprisingly deep dynamic, actually, not just about two handsome guys clashing. The tension isn't really from their canon interactions, which are basically non-existent—it's all about the conceptual parallels. We're talking about two pillars, right? Zhongli, the retired god who chose to walk away from his duty, and Diluc, who took on this self-imposed, punishing duty after his father's death and left the Knights. Their conflict isn't about the coffee vs tea rivalry meme; it's about radically opposed philosophies on sacrifice and legacy. A lot of the angst in fics comes from Diluc's relentless, self-destructive drive to protect Mondstadt clashing with Zhongli's weary, 'I've seen empires fall, young man' perspective. He's the only one who's lived long enough to truly challenge Diluc's martyr complex. The emotional beats are about Diluc being forced to confront the long-term cost of his path, and Zhongli, who thought he'd seen it all, being moved by someone's fierce, fleeting passion. It's a pressure cooker of immortal weariness meeting mortal fire.
I've read fics where Zhongli is almost paternal, trying to guide Diluc away from his own abyss, and others where it's a slow-burn romance built on mutual, unspoken respect for the other's burdens. The 'emo battle boy meets old man with too many stories' setup just works on a character level. Sometimes the fluff comes from Zhongli introducing Diluc to the simple joys he's forgotten, like a decent meal or a quiet night watching the stars.