Did John Stevens ever reveal his reasons for resigning as chief engineer of the Panama canal project?

Then on January 30, at Culebra, Stevens sent a letter to Roosevelt that reached the White House on February 12. It was six pages in length and as devoid of cant or circumlocution as all his correspondence … The letter was not a formal resignation. He never said specifically that he wanted out, only that he was not “anxious to continue in service.” He wanted a rest, and having assured Roosevelt of his high personal regard for him, he asked for his “calm and dispassionate” consideration of the matter. A reporter who talked to someone who was with Roosevelt at the time Roosevelt received the letter wrote, “To say that the President was amazed at the tone and character of the communication is to describe the feelings mildly.” The letter was sent immediately to Taft with a covering note: "Stevens must get out at once.” Even if Stevens were to change his mind, it would make no difference “in view of the tone of his letter." After a brief meeting with Taft, Roosevelt cabled Stevens that his resignation was accepted.