I always found: In the chapter Kings Cross, we discover that the “Avada Kedavra” spell did not actually kill Harry, but rather destroyed the fragment of Voldemort’s soul that was resting in Harry’s soul. Harry himself, however, could not be killed until Voldemort was destroyed.
Why, then, did he go to this “halfway point” of Kings Cross, where he is apparently given the option to “board a train” and move “on”? Surely he wasn’t dead and couldn’t die?
It wasn’t an entitlement he had to stay alive, it was simply the outcome of many factors that he hadn’t be killed. So how could he choose to move on?