Ron is the only person who truly understands how physically affectionate Harry actually is—but only with him.
Harry grew up touch-starved, so he doesn’t always know how to initiate affection, but once he gets comfortable, he craves it. He’s not obvious about it—he won’t just throw himself at people like some human-sized cat—but with Ron? He gravitates toward him.
It starts small in first year: brushing shoulders, leaning against Ron when he’s tired, sitting way too close on the common room couch. At first, Ron thinks it’s just a fluke—maybe Harry doesn’t notice personal space. But then he realizes Harry only does it with him.
By third year, it’s just normal. If they’re sitting together, Harry’s legs are slung over Ron’s. If they’re walking, Harry bumps into his side. During Quidditch practices, Harry sometimes lands and just flops against Ron for warmth, and Ron never questions it.
By fifth year, Harry doesn’t even think twice about it. He’s stressed, and the only thing that helps is Ron. So he steals Ron’s scarf in winter (Ron doesn’t ask for it back), he rests his head on Ron’s shoulder while they’re studying (Ron lets him), and he falls asleep against Ron on the couch more times than he can count (Ron just adjusts so they’re both comfortable).
Ron never makes a big deal out of it, never teases, never pulls away. And that’s why Harry keeps doing it.
By the time they’re older, it’s just how they are. They’re both in a relationship, living their lives, but if Harry slings an arm around Ron’s shoulder or falls asleep on his couch after a long Auror shift, nobody even questions it.
Because Harry needs Ron, and Ron is perfectly happy to be needed.