Why is Satan an enemy of human beings?

Soul (nafs) is a sensitive receiver of Satans delusions. In the hadith, it was indicated that there is a station in human heart for both the delusions of Satan and inspirations of angels.(1)

The following is stated in the Quran, Verily Satan is an enemy to you: so treat him as an enemy. He only invites his adherents, that they may become Companions of the Blazing Fire. (Sura al-Fatır, 6). Satans animosity towards humans begins with Adam. Satan was rejected from the Divine mercy since he had not prostrated to Adam. He became an enemy of Adam and his lineage because of this. He asked for Allahs permission to be an obstacle for them in the way going to Allah. Almighty Allah has given him that permission for testing humans and for manifestation of the abilities in their true nature. Satan says:

"Because thou hast thrown me out (of the Way), lo! I will lie in wait for them on Thy Straight Way: Then will I assault them from before them and behind them, from their right and their left: Nor wilt Thou find, in most of them, gratitude (for Thy mercies)." (al- Araf, 16–17)

Satan goes to great lengths and gives all kinds of delusions to people to have a control on them.(2) He catches some from his feeling of fear. He deceives some by useless fancies. He shows himself to some people as if he has nothing but good intentions. He causes some people to go astray by means of their appetites and some of them by means of their heedlessness The following is expressed in the hadith, He flows in human body like the blood flowing in mans veins. (3) Castles are conquered through their weak points. Satan tries to conquer man by using mans weaknesses.

People taken with Satans delusions and following him become servants and slaves of Satan instead of being servants of Allah. They become his staff by performing what he tells them.(4) Almighty Allah warns man as follows: Do not follow the footsteps of the Satan, for he is to you an avowed enemy. For he commands you what is evil and shameful, and that ye should say of Allah that of which ye have no knowledge. (al-Baqara,168–169)

Sources:
1-Tirmizi, Tafsir, 2/35
2-See Baydawi, I, 576
3-Bukhari, Bad'ul-halk, 11; Abu Dawud, Sawm, 78; Ibn Majah, Siyam, 65
4-Yazir, I, 584