Soon after, Harry heads downstairs for breakfast. Dudley looks even angrier than usual as he receives his breakfast: a quarter of a grapefruit. This is because Dudley's school doesn't make uniforms big enough to accommodate him, so the school nurse recommended a diet consisting mostly of what Vernon calls "rabbit food." Petunia insists that the entire family follow the regime to make Dudley feel better about it. As soon as Harry learned that this would happen, he sent notes to his friends. Hermione, Hagrid, and the Weasleys all sent food, and Harry received four cakes for his birthday. He eats his grapefruit happily, thinking of his cake upstairs.
Petunia's decision to enforce the diet for every family member shows that she is capable of empathy and, though what she's teaching Dudley in terms of kindness is questionable, she is doing her best to be kind. However, because Harry has suffered years of abuse at the Dursleys' hands up to this point, he sees this as just another way for them to torment him and make his life miserable--in other words, his perspective colors his interpretation of this event.
Uncle Vernon seems just as testy about the grapefruit as Dudley does. As he picks up his spoon, the doorbell rings. While Vernon is out of the room, Dudley steals his grapefruit and eats it. A minute later, Vernon returns and calls Harry to the living room. He angrily reads a letter to Harry. It's from Mrs. Weasley, asking if she can host Harry for the rest of the summer so Harry can attend the Quidditch World Cup. When he's finished reading, Vernon furiously holds up the envelope--it's covered in stamps and the postman thought it was funny.
It's clear from Vernon's reaction and Harry's thoughts that Petunia's attempts at making Dudley feel better are only making everyone more miserable--including Dudley, the one person who's supposed to benefit from this diet. This shows that while attempts like this can be made in good faith, that doesn't mean that they're always effective at accomplishing the intent.
Harry and Vernon argue for a minute and then Harry notes that he's writing to Sirius. Vernon's purple face gets blotchy and finally, he agrees that Harry can go. Brightly, Harry steps into the hallway, says to Dudley that breakfast was great, and runs upstairs. He discovers Hedwig is back and notices a second owl the size of a tennis ball flying around. Harry takes the note off of the owl's leg. It's from Ron and says that his family will fetch Harry tomorrow no matter what the Dursleys say. Harry scribbles a note back to Ron and, after adding a postscript about the World Cup to Sirius's letter, sends Hedwig off.
Harry's happiness at getting to see the Weasleys and rejoin his community reinforces that he's truly a part of the Wizarding community at this point in his life; the Muggle world holds little for him as, in Harry's estimation, it consists only of the Dursleys. This then illustrates the power that individuals have to make a community feel unwelcoming or welcoming to someone based on how kind they are.