The concept of "God" is just an easy approach to the unsolved mysteries and theories of the universe. There is too much unknown about this universe and existance in general for one to rationally hold that God has made humans in His image and differentiates us from all other forms of life. In reality, humans simply have a greater ability to make complex associations, and many people confuse this for some sort of divine consciousness or soul.
Because it is not rational to believe that we have a "soul," it should not be anymore logical to believe in an afterlife. Sure, we may think we are quite important here on earth, but this is due to our relative ignorance of the rest of the universe. Really, humans are social creatures with a great capacity for association and enhanced intelligence due to evolutions in communication such as written language.
We don't even know what is at the depths of our oceans, so it is irrational to make any assumptions as to the true nature of our existance.
As a philosophy and psychology major, these are my educated deductions. Humans are associative creatures, so people have the tendency to attach associations to things, even if they do not have all the information. This is ignorance, but it is also neccessary since it is impossible and quite inefficient to try to gather ALL information before making a simple association. Without assumptions, we would go nowhere. Thusly, it should be apparent that the concept of God or a supreme entity is a fabrication to fill a void of understanding. We WANT and NEED to have a reason for existing to satisfy our natural tendencies. "God" and the "afterlife" are relativelt accwptable justifications for this tendency. Most cannot accept that we don't...