Life is a grand opera.
From beginning to end, it's a movie.
God allowed evil to exist because it shows God's mercy. It also shows that Satan's accusations are wrong. Christ on the cross is a testament to both angels and mankind that God was right and the devil is wrong.
We live in an epic drama - a cosmic/spiritual warfare.
Imagine seeing a movie where all is fine and dandy, nothing bad happens. It's boring isn't it?
We see movies with drama, action, sadness and conflict because it keeps us on the edge of our seat, it makes us reflect after the movie is over and cheer for the main character.
Now apply that analogy to life.
We will glorify God and be in eternal bliss because of the experiences we had with sin.
A dumbbell is meant to be heavy and go against you. It strengthens muscle.
Swords are sharpened by blacksmiths in a fire.
Likewise going through tribulation and hard moments makes you stronger in spirit. That's the whole point of being a Christian. We are Satan's number #1 target. We are soldiers behind enemy lines (this world). The Bible tells us how it will end, we simply have to hold on to our dear Lord & Saviour Jesus Christ.