The New Statesman has an article that consists of a collection of statements by an array of people who doubt the existence of god or gods. It is definitely worth reading, but I recently had a not-so-subtle reminder of why I don't believe in god.
My wife was flipping channels, and came upon a documentary on PBS about life on planet Earth. The show had just begun talking about the relationship between predators and prey. During this segment, there was one scene that was only up for about five seconds, displaying a young gazelle that had been felled and was being devoured by two lions. It was still alive, looking back over it's shoulder and pathetically flailing it's front hooves at the two lions that had torn open its belly and were already feasting upon the flesh of the small animal.
A horrific scene, but necessary for the lions to continue living. Scenes similar to it are played out thousands (if not millions) of times per day, from spiders feasting on insects, to housecats toying with baby birds for sport (this happened in my back yard on Monday - I discovered a neighbor's cat had been chewing on a featherless chick near my shed), to piranha consuming other fish alive. This has been going on for hundreds of millions of years, and will likely continue until our sun runs out of fuel and expands to the point where it envelops the Earth.
Who could possibly be so naive as to believe that a benevolent, caring god would create a system that requires so much pain and suffering?
*Edited to include the link to the New Statesman article that I neglected to insert