The reason Hashem commanded us to guard our souls, and health:
Hashem created the world for a purpose, which is to provide creatures with an opportunity to recognize G-d’s greatness and to serve him through observing His Torah and Mitzvos. Now, one who is lax with his life and safety expresses that he does not care to serve G-d and does not care for His great kindness, and there is no greater belittlement and heresy than this.
The body is not yours and is a mere Pikadon in your hands:
A person does not have possession over his body at all to be allowed to hit it , or to embarrass it , or to pain it in any form of pain. [The reason for this is because Hashem gave man his life and soul as a mere Pikadon, and not for him to take possession over, and hence it is not within his jurisdiction to treat it in a way that can damage it, just as a guard must make sure to guard and prevent damage from the possessions that people entrust him with. Hence, from a certain perspective, one who damages his body is in essence stealing from Hashem.]
The body is holier than the soul:
From the fact that the Torah commands a Jew desecrate Shabbos for the sake of saving a life, we can understand that the life of a Jew is even holier than that of the holiness of Shabbos. From here we can learn the preciousness of the body of a Jew, and how he must guard it from dangers.
 Beir Hagoleh C.M. 427; See also Chofetz Chaim in Kuntrus Likkutei Imrim chapter 13.
 Admur Hilchos Nizkeiy Guf Vinefesh Halacha 4
 Radbaz on Rambam Sanhedrin 18; Likkutei Sichos Shoftim vol 34 p. 106
 Likkutei Sichos ibid
 Or Hachaim Hakadosh Shemos 31:13