Why do we struggle?

Whatever struggles we go through are only the ¹struggles 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 ¹allows struggles so we might display Him better and delight in Him more. This is for our greatest joy and His highest goal i.e. He is our greatest joy and honoring Him is our highest pursuit. 

God knows exactly what He's ¹doing (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 ¹design, no matter how it might seem the opposite at the time. God is simply calling us to trust Him more fully. 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 (and through) them. Otherwise, our struggle would eventually cease being struggles.

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 or 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). It is resting in God. 

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Footnotes:

¹struggles are the internal and organic outcome of our distrust of God. God works according to our trust or distrust. You could say how God works mirrors what we most need. The more we trust, the less internal turmoil we experience i.e. we are less anxious and upset over difficult circumstances.