Because when you do, you have to put up with stuff like this (h/t Garance):
Dubuque, Ia. — Newt Gingrich got a not-so-nice welcome here today.
As he was getting ready to leave a speaking engagement Dubuque resident Russell Fuhrman approached him in the lobby of the Holiday Inn:
"Get out now before you make a bigger fool of yourself," Fuhrman said directly to Gingrich.
Gingrich, visibly stunned, quickly moved forward to talk with other guests.
I remember hearing this story about former New York Mayor Ed Koch. I can't verify whether it's true (the Internets don't seem to have it anywhere), but the story goes like this: Not long after he left office, someone asked him what the best thing about not being mayor was. Koch replied, the other day I was walking down the street, and a construction worker yelled "Hey, Mayor Koch!" And I smiled and waved, and then he said, "You suck!" So I looked up at him and said, "Yeah? Well fuck you!" And that's the best thing about not being mayor.
Running for any office, particularly president, requires putting part of your natural humanity aside. On the trail, you're not allowed to have a range of emotions, or good days and bad days. You can't be surly, or impatient, or bummed out, as all of us are every now and then. If Newt had reacted to this guy in a way that many non-politicians would in similar circumstances -- for instance, by saying, "Thanks for the tip, jackass" -- it would have been totally understandable. It would have also violated the norms of how candidates are supposed to act and brought a storm of discussion in the news about whether he has the temperament to be president.
The ability to sustain a particular kind of upbeat mood all the time on the trail can be a function of sheer will, or it can be a function of monomania. Either way, the trail reveals whether the candidates have it. A presidential campaign is a brutal slog. Try to imagine that for the next year and a half, you almost never got a day off (and that means you work weekends, too), you had to meet thousands of people and give hundreds of speeches, and everywhere you went, even when you were just talking to one or two people on a street corner, someone was videotaping you, with your every word being recorded. Also, people felt perfectly free to come up to you and tell you what a jerk they think you are. And you had to smile and act like you like it.
UPDATE: And here's the video!