When we are no longer able to change a situation, we are challenged to change ourselves.
One of the most famous quotes in Man's Search for Meaning, is a deeply meaningful quote. If people are thrust into a situation which they are not able to change (like Frankl was when he and many others were forcibly put into concentration camps), then they must adapt in order to survive (and in some cases, thrive) -- whether that be changing their personality, their looks, etc -- no matter how challenging it is.
Everything can be taken from a man but one thing: the last of the human freedoms—to choose one’s attitude in any given set of circumstances, to choose one’s own way.
This is the crux of the book and perhaps the most important tenant of Frankl's logotherapy. That is, even in the worst of situations, people can always control their attitude. Despite having the misfortune of being imprisoned in Auschwitz and having his possessions, wife, and humanity stripped from him, Frankl had control of one very important thing: attitude. He chose not to adopt a defeated, sad attitude and demeanor. Instead, he chose to live life to its fullest no matter his circumstances.
An abnormal reaction to an abnormal situation is normal behavior.
This may seem like common sense, but given how truly abnormal and awful Frankl's situation was, it takes on a whole new meaning. When in an abnormal situation (i.e. in a concentration camp), it would be normal to be sad and beaten down -- most prisoners adopted that mindset. In order to truly survive, though, you have to adopt abnormal behavior (a positive attitude and a good outlook on life) in an abnormal situation.