In 1917, a selection of Friedrich Nietzsche's correspondence was published. In this study, we have sought to integrate this correspondence into the author's work, and interpret the letters as part of his philosophy. Is it possible to discern Nietzsche's idea of affirmation of life through his correspondence? To answer this, we have analyzed the letters the philosopher wrote to his family; the conclusions are presented below. In this correspondence, Nietzsche's interest in the inner life and the high value he places on feeling over reason are evident. Both characteristics are typical of the Romantic movement of the time, which would remain strongly evident in his letters until the end of his clear life. However, regarding the philosopher's work, E. Fink tells us that this romantic spirit mainly permeates his initial work The Birth of Tragedy (Die Geburt der Tragödie); although we see that Nietzsche himself writes in his letters of 1881 about Daybreak (Morgenröte) as being born of a human brain and heart. Regarding the content of his letters in relation to the exaltation of life over death in his philosophy, they tend to follow a similar textual pattern: initially, constant references to his poor physical (especially neurological and cerebral) and emotional health, ending with a positive and vital message. Furthermore, he always addresses his ailments (the initial negative aspects) using abundant, not serious, and even humorous irony. The positive attitudes or vital impulses in the final part of the letters fall within the Dionysian category in the author's thought, in the sense that they are primary human attitudes that have been subdued by reason. These attitudes, aligned with his idea of affirming life, he sees as genetic in his family (the fundamental essence of the Nietzsche family, he would say), and which he himself claims to possess: a highly original and personal character, strength, courage, independence, and superiority, hoping that they will prevail over his failing health and in his work. Finally, another example of his affirmation of life is found in his letters when he refers to his paid professional activity: he describes it as a constraint on his will and freedom. Besides, he enjoys small, familiar groups, but dislikes trivial gatherings and their rules of etiquette. Nietzsche may have seen this professional life as representative of 19th-century culture and heir to Socrates, where order and obedience to the law predominate, hindering the development of possibilities for creating new ways of life and new values.