Galatians 5:17 "For the flesh lusts against the Spirit, and the Spirit against the flesh; and these are contrary to one another, so that you do not do the things that you wish"
God's Word tells us to love the Lord with all of our heart, with all of our soul, and with all of our strength [Deut 6:4,5]. When you first got saved, you gave your heart to Jesus first. You didn't give Him your soul or our strength first. You gave Him your heart.
Satan will always counterfeit what God does, and do the exact opposite. He'll always go after your mind first, because he knows that if he can get something into your mind and get you to think continually about it, then he can get you to do it. That's why it's so important to realize that you CAN control your thoughts and desires.
Sin always starts in the mind. It's not sin to have an evil thought- that's temptation; but it is wrong to dwell on and entertain those thoughts. To make it simpler: A bird can lay an egg on your head, but only you can let him build a nest in your hair!
James 1:13-15 reminds us that if we think about sin long enough, we'll desire to sin. If we want to sin, we will eventually sin. After sin is done, it will leave us dead. That's why it's so important to walk in the Spirit and feed your mind with the Word of God.
Paul sums it up in Galatians 5:24,25: "And those who are Christ's have crucified the flesh with its passions and desires. If we live in the Spirit let us also walk in the Spirit." His Spirit helps you overcome evil desires!