This is one of those questions that has been eating at me for some time (yes, that was intentional). Why is it that the walking dead, the undead, zombies, zed heads, or whatever it is you want to call them, why do they look for and eat us the regular people of the world? Well, and this is just my thoughts, I believe there are two different scenarios that we should look into. The first being the "Hollywood" reasoning. The second one would be more "Scientific" in nature.
Hollywood. We seem to see a lot of zombies running around movies these days and most of them are different than their counterparts in other or older movies. We have slow walkers, mindless wanderers, zombies that can run and those elusive thinking zombies. The one thing these all have in common though is they eat people. Hollywood must have heard this from somewhere, but where?
To find that answer I look at the history of zombies in movies. Night Of the Living Dead (1968) truly is the first indication of what we think of when it comes to a zombie, but it all did not start there. Romero was influenced by others, and brought those influences together. Prior to this time we had several different genres of creatures in our hands. On one hand was the vampire, the blood sucking creature of the night. Vampires were also able to turn others into vampires with a bite. On the other hand we had stories of the voodoo zombi. This was a mindless creature that either was brought from the dead or inflicted on a living person. Usually this person would be under control by someone else.
If you take these two main themes (and add in a few other stories and movie ideas from those before) you find yourself with a mindless wandering creature that feeds on human flesh and spreads the disease through their bites. If you really want some more info on this try here: http://en.wikipedia.org/wiki/Zombie_(fictional) for they have a great history.
That is Hollywood and fiction, now let us delve into the world of science and real possibilities. Although this is not an exact science, I hope my ideas can possibly help direct the conversation on this to new ground. I also will be using the thoughts of Bob and Dave, the Madore brothers. You can find all their work here: http://necropology.com/ . I would highly suggest reading their papers before looking at my thoughts.
They mention in their papers a virus and bacterial pair that does some of the physical changes to the body. The thought I am having though is that viruses and bacteria are not that great of keeping their hosts alive. I know, they are zombies, but a large portion of the cells in the body must remain intact if it is to function at all. A cell reproduces by taking over a cell and destroying it. Usually, if the virus does not have a new cell to take over, it dies off. Bacteria can split and live a bit longer, so I think that they can continue their current role in this.
Now, we know that a virus would end up destroying it's host. There is one thing though that tries not to do that, a parasite. I think that this virus is acting more as a parasite than as anything else we would see. More than likely the virus enters the body and starts to reproduce like mad. Once the levels within the body reach a certain number though (about the time of reanimation) the virus slows or stops reproducing at those high levels. Within a body there are a lot of cells one can use, but it is no longer a renewable source. So now our virus starts to conserve cells to extend the period of time that it can remain active in the body. Add to this a long life in general and these viruses could last years within a host.
But, just like every other thing on this planet, it wants to ensure that the species continues onward. And this is where I think zombies eating people really comes in. I think the virus programs its victims to spread its seed for them. So, I become infected and am now loaded with this virus. Eventually I will die off and so will the virus, but until then I am now a moving transmitter to try and get this virus into as many other hosts as possible before I expire. The easiest way to transmit anything? by direct fluid contact. The human mouth has a ton of germs in it, and one bite can introduce those into another body. Replace those with the virus and Bam! you have a way to spread around.
Now, you may ask me one question: Why eat people? Why not just bite them? OK, that was two questions, but they have the same answer, human beings. Here is the thing, biting others is a no no to anyone not raised by wolves. But, as the stories have said, when it comes to survival of the body we will eat each other to survive. The virus just wires us into thinking that the only way we can survive is to eat other live people. This drive is now so strong that any other thought processes are now gone.
As a recap, we have a virus that acts as a parasite. In an attempt to spread itself to as many hosts as possible it creates the urge to feed in us so strong nothing else matters, creating a biting and eating mindless zombie.
I don't know about you, but that thought kind of scars me. I don't want to be some incubator for a virus. And now I find myself longing for a visit to the gun shop..... Well then, until next time everyone, stay safe.