I believe it is because of our mind and the spirit how they fight one another. They are forces that are always constantly fighting, I believe we can build up the spirit the more we get the word of God in us.
Yesterday though my mind was clouded with struggle just like everyone else sometimes has, sometimes it feels really lonely in this world, maybe you lose a friend or they don't put much effort into having a relationship with you while you try. It's part of the suffering, but we gotta keep strong and remember God loves them too, pray for them, and God loves us as well, and desires us to have peace.
Tribulations are going to come and go, like storms, sometimes storms are just simple showers. They all come by and pass. The sun starts shining again. (Mindset can say stay down but the spirit says enjoy the peace, and love, keep praying and be thankful regardless.)
Great question thank you sister.
@Psalm 27 : Just got reminded what I forgot, perfect we can be actually the word perfect in greek stands for mature. Hope this helps. We can be mature (perfect) in our faith. ( I could be wrong )