Why is Kat's death the most difficult of all for paul?

Kat's death seems to exemplify the ultimately meaningless struggle that is this war. Because Kat is so close to Paul, it shatters his world:

In despair, Paul watches as his friends fall one by one. It is the death of Kat that eventually makes Paul careless about living. In the final chapter, he comments that peace is coming soon, but he does not see the future as bright and shining with hope. Paul feels that he has no aims left in life and that their generation will be different and misunderstood. When he finally dies at the end of the novel, the situation report of the frontline states that "All is Quiet on the Western Front" highlighting the insignificance of an individual soldier during a war.