Because the serpent tempted Eve, God said that he would put hostility between the serpent's offspring and humanity.
WEB Gen 3:4 Yahweh God said to the serpent, “Because you have done this, you are cursed above all livestock, and above every animal of the field. You shall go on your belly and you shall eat dust all the days of your life. 15 I will put hostility between you and the woman, and between your offspring and her offspring. He will bruise your head, and you will bruise his heel.”
It seems then, that until that moment, there was no such hostility between us and Satan.
