It's the result of psychological trauma and it pervades all aspect of life. Flashbacks, nightmares, extreme anxiety, rage, prolonged depression, intense aversion to stimuli associated with the stress (in soldiers' cases, typically loud noises and bright lights). People struggling with PTSD are impaired socially, professionally, emotionally, or all of the above.

They're still humans capable of happiness and pleasant reflection.

but the things you might describe as "thrilling and heart racing," soldiers typically describe as terrifying and chaotic.

I implore you to ask more veterans about their experience. The near universal sentiment that I've heard is that it is simultaneously the most horrid, exhilarating, life affirming, hellish experience one can undertake. Soldiers are frequently heard to say in interviews on the subject that war is a rush unlike anything else, it makes beer taste better, colors brighter, life more valuable, etc, and yet also is combined with seeing death all around you, including those of your friends.