Life is a fight. That's not all it is, of course. Life is a gift. Life is a blessing. Life is a journey. Life is a bowl of cherries. Life is all those things, but life is also a fight.
God has a plan, and life is a fight because of where we are in God's plan. God created the heavens and earth. Human beings rebelled against God and fell into sin. God sent teachers and prophets over the ages to show us how to live. In the fullness of time, God sent his Son Jesus to redeem us from sin and reverse the curse placed upon us when we first rebelled against God. Someday the entire creation will be redeemed, and all sin, sorrow, and suffering will be no more. We live between the time of Jesus and the final redemption of creation. The eventual perfect victory over sin, death, and evil has been assured, but we are not there yet. In the meantime, there is still evil in the world, and it still causes us misery. That explains spiritual warfare. That explains why life is a fight.
The Bible tells us to be ready for spiritual warfare. The Bible tells us expect life to be a fight, and to prepare for the fight by "putting on the whole armor of God." It is a catchy phrase and a picturesque image. It also holds a truth that is often overlooked. It is called the armor of God because it is what God wears. It is called the armor of God because God himself is a warrior. God is not neutral in our struggles against sin and evil. God is not an impassive observer or a neutral referee. God is fighting on our side. So don't be naive, and don't be frightened. Don't underestimate the struggle, because it is real, but don't underestimate how much it means to have God fighting on your side.