Huh, interesting question. I'm not a hardcore Harold/George shipper myself, but I've read enough to get the appeal. It's not just about them being cute together, though that's part of it. I think the central draw is the 'forbidden' tension from the books—this intense, co-dependent friendship that's the core of the narrative, but with a rigidly defined 'no girls allowed' rule hanging over it. Fics often explore what happens when that rule breaks, either secretly or openly. The dynamic shifts from shared mischief to shared vulnerability, which feels like a natural progression.
A lot of the best stories I've seen focus on George's quieter, more observant nature balancing Harold's chaotic, performative energy. George isn't just the 'straight man'; he's the anchor, the one who truly sees Harold beyond the pranks. Fics that dig into that—George realizing his drawings are a language only Harold fully understands, or Harold's bravado crumbling when George is the only audience that matters—those hit the hardest. It takes the foundational loyalty of the books and pushes it into emotional territory the original couldn't, which is kind of the point of fanfiction, right? The ending note is often just them figuring it out, together, in their treehouse or with a new, private comic.