First off, the setting: not just Vietnam, but the very remote setting of this particular set of forces within Vietnam. The setting is isolated and oftentimes isolation is connected to insanity or delusion.
Secondly, Kiley's story is believable because it involves Special Ops soldiers. These men are known for performing the most secretive, dangerous and strenuous operations.
Because O'Brien combines isolation and stress, he is able to create a setting that could potentially break even the sweetest, most kind person--hence, MaryAnn.
As a "sweetheart," MaryAnn is characterized as this wholesome, kind-hearted and naive person, one who is unfamiliar with the stress and strain of wartime conflict. Because many soldiers, men stronger and braver than the MaryAnn portrayed, came away from the war with such severe mental trauma, a reader can easily imagine that one with less strength and fortitude could and would surely crack under these same pressures.