Matters of national security don't stress you out, do they? Because if they do, this might not be the job for you. Not only do you have to be on your game 100% of the time—no, seriously, 100% of the time; you essentially lose any and all rights to personal privacy when you join the FBI, since you can bet they're monitoring your phone calls and emails—you also have to trust your team enough to feel as though they are on their game 100% of the time. When you're tracking a serial killer, the slightest smudge of dirt or piece of hair at a crime scene could determine whether you catch the guy—we're not being sexist; most serial killers are male—or if someone else winds up dead.
If you're in a shootout, which is stressful enough on its own, you have to be able to keep your cool long enough to not only save yourself, but keep those around you safe who might be used as collateral damage. And let's not even get into the counterintelligence and counterterrorism stuff. A leaked schematic or misinterpreted intercepted message could mean countless lives. FBI Special Agents are the ones responsible for catching everyone from John Gotti and John Dillinger to Timothy McVeigh and Ted Kaczynski to President Richard Nixon during Watergate. You try investigating your President. It's stressful.