The root of his belief is the choice that man must make between a boring or painful existence, guided by a variety of the Kantian "thing-in-itself."  
His belief that the ethical spirit attempts to relieve the pain of others only because it sees all pain as its own is the subject of his The Basis of Morality.  He defined frustration as the stubbornness that results when the intellect is displaced by the determination of spirit in a series of Essays of Parerga and Paralipomena.  The root of his belief is the choice that man must make between a boring or painful existence, guided by a variety of the Kantian "thing-in-itself."  FTP, name this philosopher of pessimism, who expounded his original thesis in The World as Will and Idea.
His belief that the ethical spirit attempts to relieve the pain of others only because it sees all pain as its own is the subject of his The Basis of Morality.  
Arthur Schopenhauer