Final answer:
Krakauer's frustration upon reaching the crest of the Southeast Ridge is likely due to harsh weather conditions on Everest, which include extreme cold and low oxygen levels.
Step-by-step explanation:
The main source of Krakauer's frustration when he reaches the crest of the Southeast Ridge is likely related to harsh weather conditions, as suggested by the reference provided. Climbers on Mount Everest often face extreme cold, low oxygen levels, and low barometric pressure, all of which are hostile to human life. The strain of these conditions on the human body can lead to a range of issues, such as physical exhaustion, which may contribute to a climber's frustration.
The information given regarding the extreme cold and combat conditions experienced by soldiers in various historical contexts paints a vivid picture of the types of challenges that are also faced by climbers on Everest. For example, Krakauer may have encountered similar challenges to those faced by the 2nd Ranger Company during the Korean winter or the troops fighting in the Italian Alps during World War I.