Reasons

 There is a legend, (I don't think there's scriptural proof.) that the fight between the good angels and the bad, was over us--mankind. When Satan heard God's plan to have His Son become a human being, he objected.  Satan would not worship a Man.

Whether this is true or not, it is obvious that Satan hates us.  Since Satan hates God, then of course he would hate a mankind made in the image of God.  Also, Jesus became one of us, a man.  Aren't we blessed? And we're redeemed by Christ.  Jesus gave His life for us.

Finally, mankind, and not the angels, are heirs to the kingdom of God and eternal happiness.  What a blessing!  Too bad, Satan, you forfeited eternal happiness by your own free choice!  

My "cloistered brothers," would say, "If you can't do the time, don't commit the crime."

P.S. If Satan hates us, he absolutely loathes Mary, the Mother of God.  Makes sense, doesn't it?  She's the one who gave birth to Our Redeemer, Jesus.