Generally speaking, prayer seems rather tame to the outside observer. Prayer is most often linked to a peaceful disposition and feelings of joy and happiness.
However, prayer is called a battle by the Catholic Church.
Why is that?
The Catechism of the Catholic Church explains this in its section on prayer:
Prayer is both a gift of grace and a determined response on our part. It always presupposes effort. The great figures of prayer of the Old Covenant before Christ, as well as the Mother of God, the saints, and he himself, all teach us this: prayer is a battle.
Prayer is a battle because it requires effort.
While it is true that prayer is a gift and that we rely on God's many graces during prayer, we also need to make an effort to pray.
If we don't try to pray, we probably won't ever pray.
Enemies of prayer
This is why the Church calls it a battle, as the battle is most often against ourselves:
Against whom? Against ourselves and against the wiles of the tempter who does all he can to turn man away from prayer, away from union with God.
As the Catechism states, prayer is not only a battle against ourselves, but also against Satan and his demonic forces.
The devil does not want us to be united with God, and so he does all he can to prevent us from praying.
He will also try to distract us or tempt us during prayer, hoping that he can catch us off-guard.
In this way prayer is never truly passive, where we sit down peacefully and bask in the graces of God. While that experience is certainly possible, to achieve such peace we must battle against the numerous enemies that try to keep us from union with God.
The key is to make prayer a habit and to continually strive to battle against the forces that oppose us.
Lifelong struggle
We pray as we live, because we live as we pray. If we do not want to act habitually according to the Spirit of Christ, neither can we pray habitually in his name. The "spiritual battle" of the Christian's new life is inseparable from the battle of prayer.
As long as we breathe on this earth, we will have to battle during prayer.
Only at the end of our lives will we be able to rest in the loving arms of our benevolent Father.