The fact is that all living beings, animals and men, want to live, and this wish is paralyzed only under exceptional circumstances, such as unbearable pain or (in man) by the presence of passions such as love, hate, pride, loyalty that can be stronger than the wish to live. It seems that nature—or if you will, the process of evolution—has endowed every living being with the wish to live, and whatever he believes to be his reasons are only secondary thoughts by which he rationalizes this biologically given impulse.
