I've heard several "spiritual warfare" stories before. My pastor says "stay away from that which is dangerous," and I understand why he says that. But how do I keep away from having an unhealthy focus on such things, without completely ignoring it? Also, what exactly is spiritual warfare? Why does it happen?
I'm so glad you asked! Your pastor is right to counsel you stay away from that which is dangerous; however, when you find yourself in the midst of a battle zone and you don't have any CHOICE about being where it's dangerous, the best thing to do is to be armed and educated about how to protect yourself! A wise teacher once said that the Christian life is like living in a war zone. We don't have any choice about where to live, but we can protect ourselves in it!
What's unhealthy is to be focused on demons; what's healthy is to be focused on Christ and especially on who you are in Christ, the authority you have in Christ, and the importance of staying in moment-by-moment dependence on Christ. (Yes, it IS hard! Which is why we need to do it in the power of the Holy Spirit and not our own strength.)
Spiritual warfare is understanding that as believers, we are under attack by "the spiritual forces of wickedness in the heavenly places," and using the spiritual armor and weaponry to withstand the attacks on our faith. The above phrase comes from Ephesians 6:12, and that whole paragraph (verses 10-18) describes the spiritual armor available to us as believers.
There is an EXCELLENT book and Bible study by Kay Arthur called Lord, Is It Warfare? that I strongly recommend for you. She not only handles the scriptures well but gives good illustrations to help the reader understand what's going on.
Finally, why does it happen? Because Satan really hates God and hates God's people. He has lost the war and knows it, but he's going to inflict as much damage on God's people as he can before going down. He can't keep us out of heaven, but he can distract us from concentrating on Christ by throwing doubts about God and about our faith at us. The thing to never forget is that Satan (and his legions of demons) is a defeated foe who was stripped of all weapons by Jesus when He died on the cross (see Col. 2:15). All he has left is "wiles" or "schemes," which means he whispers lies and questions to us. The battleground for spiritual warfare is the mind, which is why it's so important to be transformed in the renewing of your mind and diligently pursue a Christian world view so you know what is true about God, yourself, and reality.
I hope this helps.