Why can't Tim forget what he has done?
In the Things They Carried, the thing Tim carries most is memory. Cross says to his friend:
'You writer types,' he said, 'you've got long memories.'
Ever since Linda's death, when he was a child, O'Brien has been carrying around the "lives of the dead." While other soldiers, like Bowker, go crazy from the burden of memory and kill themselves, O'Brien uses storytelling to make sense of the senselessness of death.
O'Brien feels responsible in the deaths of Kiowa and "The Man I Killed," and so to help him relieve the guilt, he creates fiction in which he's not the only guilty party. He creates a shared ownership in all the death. "I" becomes "we." In this way, O'Brien creates his own collective katharsis--he purges his own pity and fear of death and shares it with others.
So, to forget is to dishonor, not only the dead, but the storyteller.