 Why is Simon so unenthusiastic when the client shows up at the beginning of the story?
He sat down at his desk, passed his tongue over his teeth in distaste, groaned, fumbled in a drawer for the aspirin bottle. He looked over at Betty and said, almost as though reciting, "What I need is a vacation." "Providence," Simon told her whilst fiddling with the aspirin bottle, "will provide." Simon said, enigmatically, "Now it comes." Betty bounced up with Olympic agility and had the door swinging wide before the knocking was quite completed. Simon said unenthusiastically, "Good morning, Mr. Oyster." The client fussed himself with Betty's assistance into the seat, bug-eyed Simon, said finally, "You know my name, that's pretty good. When Simon continued to say nothing she ventured, "Time travel is impossible." "Then you'll realize that there are a dozen explanations of the paradoxes of time travel. It's my contention that within a century or so man will have solved the problems of immortality and eternal youth, and it's also my suspicion that he will eventually be able to travel in time. Simon seemed incapable of carrying the ball this morning, so Betty said, "But ... Mr. Oyster, if the future has developed time travel why don't we ever meet such travelers?" "The usual explanation, Betty, is that they can't afford to allow the space-time continuum track to be altered. If, say, a time traveler returned to a period of twenty-five years ago and shot Hitler, then all subsequent history would be changed. Mr. Oyster returned his glasses to their perch, bug-eyed Simon, but then nodded. Simon said, "You want to hire me to find a time traveler and in some manner or other—any manner will do—exhort from him the secret of eternal life and youth, which you figure the future will have discovered. Now she said, plaintively, "But where are you going to find one of these characters—especially if they're interested in keeping hid?" People would figure they had D.T.'s." If you could go back a few thousand years, the things you would wish to see would be a Roman Triumph, perhaps the Rites of Dionysus, or one of Alexander's orgies. "No ma'am, you'd have to stick to some great event, both for the sake of actual interest and for protection against being unmasked." You can take the plane to Munich, spend a week there and—" Simon was shaking his head.
 He knows he cannot accomplish what Mr. Oyster is asking, and knows he will have to turn him away.
 He knows Mr. Oyster's reputation, and does not want to get involved in his affairs.  He knows he cannot accomplish what Mr. Oyster is asking, and knows he will have to turn him away.  He only takes on easy cases that he knows will pay the bills.  He has too much of a headache to deal with the new client.