Why did harry become a horcrux?

In the Harry Potter series, Harry Potter became an unintended Horcrux of Lord Voldemort due to a series of events and the nature of Horcrux creation.

When Voldemort attempted to kill baby Harry with the killing curse (Avada Kedavra) at Godric's Hollow, the curse backfired. This was because of the protective charm cast by Harry's mother, Lily Potter, who sacrificed herself to save her son. The backfiring of the curse caused a piece of Voldemort's soul to split off and inadvertently attach itself to Harry, who was the only living being present at the time123.

This event occurred because the act of murder, specifically the cold-blooded murder of Harry's parents, had already torn Voldemort's soul apart. When the killing curse backfired, it further fragmented his soul, and a piece of it latched onto Harry. This created an unintended Horcrux within Harry, linking him to Voldemort and explaining their psychic connection, Harry's ability to speak Parseltongue, and the pain Harry felt in his scar whenever Voldemort was near or active123.