Everyone knows a survivalist who's absolutely certain they'd do well in a Zombie Apocalypse. Unfortunately, fighting and surviving can be a lot harder than it looks in the movies. Even though some people know how to survive in the wilderness and fight with a knife or aim with a gun, that doesn't necessarily mean that just any person will be able to live that kind of life. There are lots of factors other than one's own survival that most people don't take into account when dreaming of the day they get to kill zombies, such as the fact that some of those zombies may be their loved ones.
Environmental factors
First, let's discuss the biggest problem in the event of zombies: mosquitoes. We've all seen the movies where just a drop of zombie blood in a cut or in your mouth is enough to turn you. If mosquitoes can infect people with malaria, they can infect people with the Zombie Virus in a heartbeat. Then, there's the weather. Depending on the area of the country you live in, you could be faced with heavy rains, tornadoes, hurricanes, earthquakes, or heavy snow, all of which throw a wrench into your survival plans. If you don't have a solid shelter, these conditions would almost certainly mean your end. Also, who knows if the virus could infect other animals as well? You don't want rabid birds attacking you in flocks, and since the zombie virus is fictional, we don't know how it would affect the creatures around us.
Personal factors
Imagine your grandmother has fallen and broken her hip. She's bedridden in the hospital until she heals. Well, when the zombie apocalypse comes, she's either left completely abandoned in the hospital with all the other sick people, or she's zombie food and will eventually have to be put down. She might even kill a few people first, dragging her broken body around looking for flesh to gnaw on.
If the zombies ever come, there's a good chance you'll have to shoot your loved ones in the head. This includes: small children, the elderly, the disabled, and the inconvenienced (such as those with asthma). Also, anyone who relies on prescription medicine to stay healthy or sane, because who's going to provide them with their pills now that most of the country's dead?
Societal factors
The future that you had planned, whether it be having a spouse and kids, seeing your work published, or just retiring on a beach somewhere, is now gone. If there will ever be a time when the dead don't walk the Earth and seek to kill, it will be hundreds of years from now. The infection spreads too easily and rapidly to be completely contained, and we would never be sure that we've gotten every last one. In short, a zombie apocalypse would steal away everything and everyone you've ever loved.
