Dan slips away for a moment during his outdoor wedding with Joy, sees that Socrates' business card now reads Happiness, and feels his teacher is present everywhere—got it, great, but what does this ending express about the novel and its philosophy?
Well, it's definitely an ending that brings firm closure to the book by giving us a direct message: mission complete, Dan.
All through the novel up to this point, Dan has wondered if he's really attained happiness yet, or a warrior's state of mind yet, or enlightenment yet. Going through the gate (see Symbols, Imagery, Allegory) was a profound vision that convinced Dan he surpassed his fear of death—but then again, Socrates taught him again and again that visions and feelings and thoughts come and go and can't be trusted.
So what if Dan's fear of death were to return? What if his training and discipline were to fade away once more, as when Socrates sent him away for years? Has he really succeeded?
But during the ending, in this private moment of Dan's wedding, the teacher's business card affirms the student's happiness. (Because in this book, if Socrates says something, it must be true.) Dan is delighted by the changed card. He feels “the truth” (E.34). Socrates has not abandoned him. Socrates is one with everything; he is the beautiful trees and clouds nearby, and all of this nature is Socrates saying, "Yes, Dan, you did it, you are happy at last."
Then check out the very last two lines (E.35). “Socrates was here. He was everywhere.” We get the italics emphasizing: It was real. The strange man jumping on the gas station rooftop unbelievably and giving you all those odd visions—all of that happened, maybe not literally, but in a true enough sense. And those truths will never forsake you, Dan, because Socrates and what he taught you is everywhere you will go; they will always be with you.
In short, Dan won. He did it. He's happy. That's what the ending is saying. Aw, shucks.