Frankl argues in Man's Search for Meaning that having a sense of purpose gives people happiness. Unlike Nietzsche's will to power or Freud's will to pleasure, Frankl believes that people are primarily driven by a deep desire for meaning in life. He calls this idea of a will to purpose logotherapy.
To illustrate this concept, Frankl recounts his time in Auschwtiz, where in the worst of all situations—a situation designed to rob life of its meaning—those who could maintain a sense of meaning had a slight edge in survival. He also emphasizes that who lived and died in the camps was predominately random. However, those who could appreciate moments of hope, no matter how tiny, and keep their minds fixed on a purpose, at least had a chance of surviving.
Frankl recounts that thinking about the joyous reunion he would have with his wife after they were both released gave him a goal and purpose in the lager. (He stresses that love is an emotion that infuses life with meaning). Even though his wife was quickly killed in the camps, Frankl didn't know this, and so this picture of wanting to survive to reunite with her gave him hope.
Without a strong sense of purpose, people succumb to despair and are more likely to die, Frankl argues.