In Man's Search for Meaning, what are the main ideas of logotherapy, and how were they proven in Frankl's experiences at the concentration camps?

At the heart of Frankl's logotherapy is the idea that humans are driven not by a sex drive or by a desire for power but by a desire for meaning or purpose in life. If we feel our lives have a purpose, we are more likely to survive and thrive. If we feel, however, that life is meaningless and we have nothing to offer, we are likely to wilt and be driven to unhealthy and self-destructive behaviors.
In the concentrations camps, most notably Auschwitz, Frankl observed that the men who lacked a sense of purpose often gave up hope and died. Frankl—and this must be strongly emphasized—is very, very clear that the primary reason that any one person was more likely to survive in such an environment was luck. He makes it clear that optimism alone was not going to keep anyone alive.
However, he argues that having optimism and a sense of purpose could help give someone an edge in surviving. Frankl said that, in part, he survived because he was able to keep focusing on seeing his wife again after he got out. The desire to be reunited with her gave him a strong sense of purpose that drove him to try to stay alive. Ironically, she was killed in the camps, so he never did see her again, but the idea of seeing her nevertheless infused his life with meaning.