Help with Questions to All Quiet On The Western Front: What have Paul and his friends lost

Later in the story, Paul writes that “Their stillness is the reason why these memories of former times do not awaken desire so much as sorrow—a vast, inapprehensible melancholy. Once we had such desires—but they return not. They are past, they belong to another world that is gone from us,” (121).

What have Paul and his friends lost? Innocence? Love? The ability to remember their prior lives? Their sense of humanity?