Why did Harry Potter see the Thestrals

Why couldn't Harry see Thestrals at the end of Goblet of Fire?

Q: In the fifth book, Harry can see the Thestrals. Can you?

A: Yes, I definitely can. That's a really good question because it allows me to clear up a point. The letters I had about the Thestrals! Everyone told me that Harry saw people die before he could see the Thestrals. Just to sort this out once and for all, this wasn't a mistake. I would be the first to say that I made mistakes in the books, but it wasn't a mistake. I've really thought this through. Harry didn't see his parents die. He was one year old and in a cot at the time. Although you never see this scene, I wrote it and then edited it. He didn't see it; he was too young to appreciate it. When you learn about the Thestrals, the only way to see them is if you really understand death in its broader sense and if you really know what it means. Someone said Harry saw Quirrell die, but that's not true. He was passed out when Quirrell died in Philosopher's Stone. He didn't know until he came around that Quirrell had died when Voldemort left his body. Then you have Cedric. A fair point with Cedric. Harry had just seen Cedric die when he got back into the carriages to return to Hogsmeade Station. I thought about it at the end of Goblet because I knew from the start what the carriages were drawing. I knew what was there from the Chamber of Secrets, where wagons of invisible things are pulled. I decided it would be strange to do something at the end of a book. Anyone who has been bereaved knows that shock is immediate, but it takes a while to fully realize that you will never see that person again. Until that happened, I didn't think Harry could see the Thestrals. That is, when he went back he saw these creepy things. It set the tone for Phoenix, a much darker book.