Buddy spends most of "Raise High the Roof Beam, Carpenters" and all of "Seymour: an Introduction" looking at most people as though they were, in the words of The Catcher in the Rye's Holden Caulfield, a bunch of "stupid morons." The Matron of Honor is irritating, Buddy's colleagues have "academicitis," his students are hopeless, and the general reader will probably miss his point entirely. He dreads going into room 307 tomorrow, where he'll have to try (yet again) to impart knowledge on a generally impossible group.
And then, somehow, we end up with this very surprising passage:
I know […] there is no single thing I do that is more important than going into that awful Room 307. There isn't one girl in there, including the Terrible Miss Zabel, who is not as much my sister as Boo Boo or Franny. They may shine with the misinformation of the ages, but they shine. This thought manages to stun me: There's no place I'd really rather go right now than into Room 307. ("Seymour" 9.8)
This is a huge tonal switch from what we've seen in the rest of this story (definitely check out "Tone" for examples and a more complete discussion). Where we once saw sarcasm and cynicism, Buddy is suddenly optimistic and even reverent. This ending is similar to that of Franny and Zooey, the Glass family short story duo that came out a few years before this one. In Franny and Zooey, Franny Glass undergoes a spiritual crisis, and spends a lot of time complaining, much as Buddy has done here, that everyone else is selfish, petty and doesn't "get it." The book ends with Zooey convincing Franny that everyone – even the pretentious professors and the snobby co-eds at her school – is worthy of love. Everyone is Jesus Christ, he says, and she should treat each person all accordingly. Of course, Zooey is passing on the wisdom courtesy of Seymour.
And this is exactly what Seymour has taught Buddy in this book. Think about Seymour's interpretation of the passage from the Bible when Christ says to call no man fool (see 2.1.15). It's because no man is a fool at the end of the day, says Seymour. Ultimately, every person is worthy of love simply by virtue of being a person.
We know that Seymour is the ultimate guide and ultimate guru (see "Character Roles"), for Buddy and for the rest of the Glass family. Buddy makes it clear that Seymour is the source of this final lesson. Right before Buddy reveals this revelation, he says: "I can't finish writing a description of Seymour […] without being conscious of the good, the real. This is too grand to be said (so I'm just the man to say it), but I can't be my brother's brother for nothing" ("Seymour" 9.8). He even adds, at the end of his philosophizing, "Seymour once said that all we do our whole lives is go from one little piece of Holy Ground to the next. Is he never wrong?" ("Seymour" 9.8).
What's interesting is that Buddy learns from Seymour even after Seymour's death. In fact, Buddy learns this final lesson through the process of writing about Seymour. This is an important concept. Buddy isn't just writing here; he's discovering, exploring, and learning as he goes. That's why this process is so hard for him. Buddy isn't just struggling to explain Seymour, but struggling to understand him and their relationship. What's more is that Buddy has an epiphany or two in the process. (See his "Character Analysis" for more on just what Buddy learns from all this literary wandering.)