In a lot of ways, Nick's dad is a stock father figure. Instead of playing catch, he just takes his son along for some, shall we say, less conventional father-son bonding instead. But as if his fatherly-ness didn't make him authoritative enough, he's also a doctor. Coupled with the fact that he also white in a group of people who are non-white, and a male operating on a female (see Nick's "Character Analysis" for more on these ideas), it's pretty safe to say that Nick's dad comes off as someone who is probably more used to being listened to than doing the listening. It all comes down to this idea of control over a situation—control that, as Hemingway shows us, can easily devolve into a complete lack of control.
We're not saying that Nick's dad is a bad guy. Even though he's pretty stern with Nick in the beginning, he's also very remorseful at the end. The fact is, he's the character in the story who really gets set up for defeat. Think about it: if a character is going to fall, that means that they have to be pretty high up already, right? That's one of the main tenets of tragedy going all the way back to the Greeks.
Nick's dad is in a pretty good position to be that tragic person. He is in an authoritative position with his son and with the Indians at the camp, and even with society as a whole (being a doctor and all). And after the operation, he's all elated over his success—"feeling exalted and talkative as football players are in the dressing room after a game" (38).
But that high doesn't last for long. In fact, the crash is pretty hard. The before and after change undergone by Nick's father is pretty drastic. Not only is "all his post-operative exhilaration gone" (47), but now he has to explain the sobering facts of suicide—and their implications of masculine weakness—to his kid. His original intent, which probably had to do with appearing in a certain light in front of his son, goes completely awry. Tragic? We think so.