Riding roller-coasters usually (unless you are psycho) induces a release of adrenaline and other chemicals, like endorphin (your bodies pre-emptive effort to combat any possible pain and stress you may be about to experience), into your body. Endorphin is a strong chemical, specifically released to make you feel relaxed and confident. Obviously this explains the natural high you may get from riding a coaster.
However, every now and then you get that person who ruins it for everyone by having a heart-attack. This person's body was not conditioned for this type of stress, and clearly they did not enjoy their ride much. Elevated blood pressure,
erratic heartbeat and palpitations...they should have instead taken a ride on the log flume. For the rest of us thrill-seekers, this type of gung-ho, take-all-comers attitude gives us a confidence boost by wiring our brains to not be overly stressed in actual emergencies.
Imagine the people out there who have grown up in small towns. You may have heard how such people can have a difficult time adjusting to the hustle and bustle of city life, especially in larger cities. This has to do with everything from their metabolism and diet, to their sleep cycle. Their body simply needs to get used to the new environment. The same goes for new combatants. Military's try their best to simulate combat for new trainees. The better the simulation, the better prepared people will be for combat. The first few combat situations for people are obviously going to be the most dangerous, being a truly foreign experience for most peoples bodies.
Physiologically it's going to push your body to the limit. After all, you sense your life is in serious danger. But as many people deal with this stress, they can start to acclimate until they ideally stay fearful, but become extremely focused on the fight. The adrenaline rush (and again, other chems) is part of what you need to get used to. If your body has a naturally stable system when exposed to these chemicals, or you have trained it to be so, then you are at an advantage compared to your peers.