Quite possibly one of the most trustworthy resources here would be <PERSON>. His explanation of the motivation for eating humans follows: source (em. mine):
Motivation
Why do the undead prey upon the living? If it has been proven that human flesh serves no nutritional purpose, why does their instinct drive them to murder? The truth eludes us. Modern science, combined with historical data, has shown that living humans are not the only delights on the undead menu. Rescue teams entering an infested area have consistently reported them stripped of all life. Any creatures, no matter what their size or species, will be consumed by an attacking zombie. Human flesh, however, will always be preferable to other life forms. One experiment presented a captured specimen with two identical cubes of meat: one human, one animal. The zombie repeatedly chose the human. Reasons for this are still unknown, but one theory for human preference is that humans are the only creature that reanimate and seek out new victims, allowing the virus to spread to new hosts, as any other creature infected with the virus will simply die. What can be confirmed, beyond any shadow of doubt, is that instinct brought on by Solanum drives the undead to kill and devour any living creature they discover. There appear to be no exceptions.
Concerning intelligence, <PERSON> further elaborates on this point (em. mine):
Intelligence
They do what they are programmed to do, nothing more. 44
Imagine a computer programmed to execute one function. This function cannot be paused, modified, or erased. No new data can be stored. No new commands can be installed. No new equipment will be added, and if any are lost the computer will simply continue to execute it's function without as if it never had it. This computer will perform that one function, over and over, until its power source eventually shuts down, recognizing and using only the tools and equipment still currently attached to it. This is the zombie brain. An instinct-driven, single-minded machine that is impervious to tampering and can only be destroyed.
So it seems that the answer is two-fold, according to <PERSON>. Zombies are in a sense programmed to do one thing, and one thing only (much like common viruses we see every day). That thing is spreading the virus to the most capable organism, in this case, humans.
Bonus Round!
The primary motivation behind everything above is the virus itself (which at this point, is the zombie), and the natural tendency of that virus to reproduce:
Solanum works by traveling through the bloodstream, from the initial point of entry to the brain. Through means not yet fully understood, the virus uses the cells of the frontal lobe for replication, destroying them in the process. During this period, all bodily functions cease. By stopping the heart, the infected subject is rendered "dead". The brain, however, remains alive but dormant, while the virus mutates its cells into a completely new organ. 209