Enemy is someone or something that's hostile to us. If someone is being unfriendly, aggressive and trying to hurt us in any way, then he/she is an enemy. If something (animal/ nature) which brings no benefit but harm to us, then it's an enemy to us. If we mistreat ourselves like hurting and lying to ourselves and even giving up on ourselves. We are our own enemies.

Here is a dictionary definition of the word enemy en·e·my (n-m) n. pl. en·e·mies 1. One who feels hatred toward, intends injury to, or opposes the interests of another; a foe. 2. a. A hostile power or force, such as a nation. b. A member or unit of such a force. 3. A group of foes or hostile forces. See Usage Note at collective noun. 4. Something destructive or injurious in its effects: "Art hath an enemy called Ignorance" (Ben Jonson). adj. Of, relating to, or being a hostile power or force. [Middle English enemi, from Old French, from Latin inimcus : in-, not; see in-1 + amcus, friend.] Synonyms: enemy, foe, opponent These nouns denote one who is hostile to or opposes the purposes or interests of another: betrayed by enemies; a foe of fascism; a political opponent.

Hi... could be people but also nature...

The enemy is yourself.

Someone who is not being your friends for some reason like he/she/you doesn't like for each other