Stress
Most of the time, you're plopped on your butt, driving. Hey, at least you won't be on your feet in uncomfortable shoes all day. There will be no itchy uniform and no annoying co-workers. Just don't think about the really, really dangerous stuff you've got in your truck, and what could happen if you get into an accident. Otherwise, you might start to stress.
In this career, you're never just cruising down the highway while singing along with Taylor Swift. This is a high-security job. Having a good day means making it to your drop-off point safely and securely. Having a bad day means having to defend your truck—possibly with lethal force.
If someone decides to rob a courier, it's not going to be just some Joe Shmoe who's down on his luck and decides to rob a nuclear transport vehicle for some sandwich money. It's going to be someone with a well-thought-out and highly devious plan; someone who will stop at nothing to steal nuclear materials. Knowing that those are your adversaries can be...a little scary.
If you're the gentle type, it gets scarier, because you might have to use firearms to defend your cargo. Sure, you'll be acting for the greater good, but it's still not an easy thing to do. If you had a hard time saying goodbye to your goldfish (RIP, Sunny), then this might not be a good career for you.