Why do we struggle?

Whatever struggles we go through are only the struggles God knows we need to go through to refine us, draw us closer to Him, and make us more like His Son (Rom 8:28-29). He allows struggles so we might display Him better and delight in Him more. This is our greatest joy and highest goal. 

God knows exactly what He's doing (and allowing). Nothing is by accident or catches God off guard (though it may catch us off guard).

The why, how, and when of our struggles are by God's wise and loving design, no matter how it might seem the opposite at the time. God is calling us to simply trust Him. This is the purpose behind everything we experience. Whether our experiences are good, bad, or ugly, the end goal is always the same i.e. His highest glory which is also our greatest good and joy.

The very fact that we struggle with the things we do is because there are lessons yet to be learned from them. Otherwise, the struggle would stop.

As we learn those lessons, the struggle subsides. There is less internal turmoil and greater calmness— even though the outer turmoil (circumstances) that cause us to struggle may not change.

We experience a peace that is beyond reason (it "passes understanding" Phil Phil 4:6-7), i.e., a peace that doesn't make sense in light of our circumstances. This is the essence of contentment (Phil 4:10-13).