Seamus didn't realize Dean was his best friend until it had been an established fact in the eyes of all others for nearly two and a half years. He didn't even work it out for himself, either. He might not even have know were it not for Lavender.
It was about half way through third year. Lavender was calmly sitting in the common room, minding her own business, when Seamus stormed through the portrait hole with all the fury of a man who's had his transfiguration notes tossed in a mud puddle. He stood for a moment, fuming at the comfortable but seemingly empty room, before noticing poor little Miss Lavender Brown curled up with her divination book. Sensing a captive audience, Seamus embarked on a furious tirade against Slytherins in general, and Vincent Crabbe in particular (peanut brained, evil, disgusting mindless zombie), with a fervor that would have done even Ron proud.
About seven minutes into Seamus' monologue , Lavender decided she could take no more and broke in with an exasperated, "Why don't you tell Dean? He's your best friend, he's probably used to your insanity,"
"What do you mean?" he asked, stunned.
"Well, Seamus, my darling dearest wonderful friend, I've been wondering for the last forty-six seconds exactly how long it will take before you start foaming at the mouth," she said as pleasantly as she could, confused by his puzzlement.
"That's not what I meant," he said, brushing off the mild insult like a cobweb. "What you said about Dean-"
"Well," she replied, really and truly bewildered, "This is the kind of thing people usually discuss with their friends, and while I do generally like you as a human being, we're hardly attached at the hip like you and Dean."
Without even bothering to reply, Seamus stumbled off to turn this revelation over in his mind.
Friends? He supposed he and Dean were friends, they certainly spent nearly all of their time together, but weren't friends supposed to get along? And the prolonged amounts of time together, that was mostly circumstance. McGonagle had said it, hadn't she, your house becomes like your family, and it was doubly true for your year They were the people you spent nearly all you time with, and there were pretty limited choices.
Harry, Ron and Hermione were always off being heroic together, Neville had his plants and his perpetually missing toad, and Lavender and Parvati were finishing each others' sentences before the first week was up, and that just left Seamus and Dean, thrown together pretty much all the time.
Because when you spend lessons and mealtimes with a person, it's only natural to walk to meals from class with them, and then it makes sense to finish your dinnertime conversation walking back up to the dorms, where you naturally fall into doing your homework together, and before you know it you're hanging out in the common room long after everyone's asleep, bickering over the relative merits of muggle versus wizarding sports as the fire burns away to nothing.
Seamus had always believed, for some reason, that friendship was built around shared interests, mutual respect, or compatible personalities. This is why he was blindsided by the notion that Dean was not only his friend, but his best friend. Dean and Seamus fight about everything, all the time, ranging from a steady stream of gentle bickering to the type of shouting match which makes the entire Great Hall turn and take notice.
They even dueled once, in second year, about who the Heir to Slytherin was. They'd ended up both of them with fractured ribs, various bruises, and a healthy respect for their own and each other's abilities to do harm, and any grudge between them was forgotten within days.
This is how Seamus explains to himself the fact that it's taken him years to realize the obvious truth; that the boy he spends all his time with, misses every holiday, and sits next to at every single lesson, is, in fact, his best friend.
Besides, he rationalized, it had all happened so gradually he had hardly noticed anything changing until that day when he finally realized that Dean had changed in his mind from "that quiet kid who sleeps across the dorm" into someone unique, a staunch defender of muggle football, worthy dueling partner, brilliant artist, and integral part of Seamus' day to day life.
The realization wasn't shattering, in fact, it changed nothing. Seamus decided the subject was something to consider later, and ran down to meet his best friend at dinner.
