Neville Longbottom can see Thestrals because he witnessed the death of his grandfather when he was a young boy. In the Harry Potter series, it is established that only those who have witnessed and fully comprehended the death of another person are able to see Thestrals. This ability is a reflection of the character's emotional maturity and understanding of mortality. Therefore, Neville's ability to see Thestrals is a testament to the hardships and loss he has experienced in his life. **

In part 4 of the Harry Potter series, Harry cannot see the Thestrals because he has not yet processed the death of Cedric Diggory. The ability to see Thestrals is linked to witnessing and understanding death, and it is only after Harry has had time to come to terms with Cedric's death that he is able to see the Thestrals. This shows that the ability to see Thestrals is not just about physically witnessing death, but also about emotionally comprehending it. **

Harry can't see the Thestrals since birth because he was too young to understand and process the death of his mother when he witnessed it. The ability to see Thestrals is not just about witnessing death, but also about comprehending and accepting the concept of death. Similarly, Muggles who have seen death cannot see Thestrals because they do not have the magical awareness and understanding required to perceive these magical creatures. It is not just about the physical act of witnessing death, but also about the emotional and mental understanding of it. **

Harry only sees the Thestrals after Cedric's death because witnessing death is a requirement for being able to see them. Since Harry did not fully comprehend Cedric's death until he had time to process it, he was not able to see the Thestrals until then. The Thestrals are magical creatures that are only visible to those who have witnessed and understood the finality of death. **

Harry couldn't see the Thestrals at the end of the fourth year because he had not witnessed death until Cedric Diggory died during the Triwizard Tournament. The ability to see Thestrals is only granted to those who have witnessed and comprehended the reality of death. Since Harry was in shock and did not fully process Cedric's death at the time, he was unable to see the Thestrals until the following year. **

Thestrals are a magical creature in the Harry Potter series that are described as skeletal, winged horses with reptilian features. They are only visible to those who have witnessed death and have processed the experience. In Harry Potter and the Order of the Phoenix, Harry is able to see Thestrals for the first time after witnessing Cedric Diggory's death at the end of the previous book. Thestrals are used to pull the carriages that transport students from the Hogwarts Express to the school. **

At the end of Book 4 in Harry Potter, the Thestrals are used to pull the carriages that transport the students back to Hogsmeade station after the end of term. Harry, who can now see the Thestrals due to witnessing Cedric Diggory's death, is able to see them for the first time and is surprised by their appearance. The Thestrals continue to be used to transport students to and from the Hogwarts Express in subsequent books. **

Harry Potter only sees the Thestrals after the death of Cedric Diggory because he has witnessed and processed the trauma of death for the first time. The Thestrals are magical creatures that are only visible to those who have witnessed and comprehended the reality of death. As a result, Harry is able to see the Thestrals after witnessing Cedric's death during the Triwizard Tournament. This event marks a significant shift in Harry's understanding of mortality and loss, allowing him to perceive the previously invisible creatures. **