When one thinks of Harry Potter's friends the names that immediately come to mind are Ron and Hermione. If you keep pressing fans might tack on Ginny, Luna, and Neville. Maybe you might even add his other dormitory mates, Dean and Seamus. But what about Cedric Diggory?
Hogwarts champion in the Triwizard Tournament. Hufflepuff seeker. Hufflepuff Prefect. When you think about it Cedric and Harry were really more rivals than friends, forced into contact with one another by circumstances outside either of their control. They were certainly important to one another, and they absolutely went through important and life-changing events together. But they were not really friends.
Girl Troubles
It's important to have things in common with your friends, like how Ron and Harry both love Quidditch, or how Hermione and Harry could relate through their experiences in muggle households. It is not helpful though to favor the same girl as someone who is supposed to be your friend.
Unfortunately, that's exactly what happens to Harry and Cedric. Cho Chang is beautiful, a Ravenclaw, and one year older than Harry, as well as one year younger than Cedric. When Harry learns Cedric has already asked Cho to the Yule Ball he is devastated. His feelings toward Cedric are anything but friendly.
Quidditch Foes
Cedric is a good person, there is a reason he was sorted into Hufflepuff house. When Hufflepuff and Gryffindor face-off in the first Quidditch match of book three, Harry Potter and the Prisoner of Azkaban, Harry takes a tumble off his broom due to the presence of dementors.
Rival seeker, Cedric Diggory, catches the snitch. Cedric, after realizing what had happened to Harry, offers a rematch, which is turned down. That's a kind gesture, but that's not because he and Harry are friends. It's because Cedric is a good person and would have offered no matter his opponent.
Forced Proximity
Harry knew of Cedric before the Triwizard Tournament in Harry Potter and the Goblet of Fire, but they didn't spend time together. They competed on the Quidditch pitch, they passed in hallways, Cedric admired Harry's Firebolt.
But, even with Quidditch in common, they did not spend time together until they were both chosen as Triwizard Champions and thus forced to do so. They are acquaintances, certainly, but not friends.
Houses and Classes
Dues to class schedules it can be very difficult to form bonds at Hogwarts School of Witchcraft and Wizardry with a student who is both not in your house and not in your year. Cedric and Harry were in Hufflepuff and Gryffindor respectively and were two years apart at school. They didn't share dorms, common rooms, meals, practices, or classes.
Most cross house friendships we see in the Harry Potter series are across house lines, but with students in the same year. Like Harry and Cedric, different students in both houses and years don't spend enough time together most often to create firm bonds.
Triwizard Tournament Alone
Fans of the Harry Potter series do get to see Cedric and Harry in one another's company quite a few times, but it always surrounds the Triwizard Tournament.
They meet up after their names come out of the Goblet of Fire, they see each other at tasks and photo calls, but this doesn't ever bring Harry and Cedric into each other's company between official engagements for more than a quick word. That's not really a full friendship.
Name in the Goblet
Cedric is not mean spirited. He is genuinely kind to Harry throughout their competitive endeavors in the Triwizard Tournament, but he does ask Harry how he managed to get his name into the Goblet of Fire.
While he doesn't go off the deep end as Ron does, when Harry tells him he didn't put his name in, Cedric still doesn't act as if he really believes that Harry didn't put the name in himself. Trust was not something Cedric and Harry ever built up between them.
Amos Diggory
When Amos Diggory comes to see his son perform in the third task of the Triwizard Tournament, he also takes a moment to berate his competitor, Harry Potter, about how he has usurped his son and his glory, as Rita Skeeter keeps leaving Cedric out of articles, as if Harry is the only competitor from Hogwarts. Cedric kindly tells his father to lay off but, if Harry was really Cedric's friend, odds are Mr. Diggory would have heard all about Harry already and would know better than to accost him on school grounds.
Potter Stinks
In both the book and the film Harry Potter and the Goblet of Fire "Potter Stinks" badges, that also read "Support Cedric Diggory" are seen around Hogwarts, including on the robes of Hufflepuff friends of Cedric.
Cedric does tell Harry he has asked his mates not to wear them. But, if Harry's friendship meant that much to Cedric, one would assume to "real" Hogwarts champion would have been able to convince his compassionate Hufflepuff cohort not to wear them. He couldn't have tried terribly hard.
Quidditch World Cup
Harry, Hermione, and the Weasley's use a portkey to get to the Quidditch World Cup. They share their transportation with Cedric Diggory and his father, Amos.
Again, if Harry and Cedric were real friends, as they are both quidditch seekers and huge quidditch fans, one would assume, once at the World Cup campgrounds, they might have run into one another, or made a point to get together to discuss the match. That's not something that happened.
Don't Discuss the Tournament
Even though the Triwizard Tournament is the even that threw Harry and Cedric into one another's lives fans never see them building a stronger bond because of this random connection. Harry offers Cedric information on the first task out of a sense of fair play and Cedric offers Harry clues as to how to use the golden egg before task two again, out of a sense of fair play.
But neither of the boys seek one another out to discuss the tournament, how it feels to be alone, as one of four individuals. They don't build a relationship on top of their acquaintanceship.