As a psychologist and concentration camp survivor, Frankl writes his book to
try to answer this question: How was everyday life in a concentration camp reflected in the mind of the average prisoner?
Frankl wants to analyze how the average prisoner, like himself, survived. He excludes those prisoners, such as the kapos, who survived because of the extra privileges they earned for being willing to brutally abuse other prisoners. He also excludes those who managed to find higher status positions, such as doctor to other inmates.
By looking at the vast bulk of prisoners who had no special privileges to help them survive, Frankl is able to discern that, although luck was a huge factor, those prisoners who had some compelling reason to stay alive fared best. He came to understand from his experience at the camps a life principle that can be applied anywhere: we may not be able to change the conditions of our existence, but our attitude towards them can make them more or less bearable.
Those whose felt their lives had purpose and meaning were better able to hold up in extreme conditions. Frankl quotes Nietzsche when he states,
He who has a why to live can bear with almost any how.
Frankl himself found meaning in both talking in his mind to his wife and focusing on staying alive in the hopes of being reunited with her. He noted carefully how other individuals responded and realized that those who had a similarly positive attitude were in a far better position than those who had given up. He wrote his book because he felt his insights had great value to people in general, not just camp survivors.