Answer:
In "The Things They Carry" O'Brien was referring to the love between Jimmy and Martha
Step-by-step explanation:
Jimmy has been in loved with Martha since they were younger and before he went to war, he didn't want to make Martha suffer and he burned all the letters so he wouldn't have to carry all that love with him in war, then when he sees her again, he tells her that he loves her, and even when she never got married she does not say anything back, then O'Brien wants to help Jimmy writing a story in his book to get Martha back.