One thing in common with all atheists is disbelief in the supernatural. This disbelief stems from the fact that strong evidence (multiple objective observations of repeatable events) points to naturalism while weak evidence (personal testimony of a single event) points to supernatural explanations. Life after death and the existence of an immaterial soul (two similar claims) are supernatural claims that exhibit such weak evidence.
The main reason for dismissing the idea of life after death is that all the evidence about human beings points to our being biological animals rather than embodied spiritual souls. Consciousness is still somewhat of a mystery to us, but we do know that it relies on brain activity. With no brain, there is no consciousness.
All the experiences, sensations, and emotions we have are dependent on the activity of certain parts of the brain, and when one of those parts is damaged, its respective aspect of experience is altered or hindered for the individual. And if you stimulate certain areas of the brain, you get an involuntary change of experience (if you stimulate the part of the brain that controls humor, everything becomes funny). In fact, damage to certain parts of the brain has been seen countless times to completely change someone’s personality forever. And when someone’s brain stops functioning altogether, they will stop giving any signs of consciousness. Consciousness is the center of our experience, and still having it after death goes against all we see.
Consider if we didn’t need a brain for consciousness. If this were true, then there’s no reason why any object couldn’t be considered conscious, and there’s no reason for us to have bodies in the first place. And as far as near-death experiences, their very name – near-death – immediately disqualifies them as evidence for life after death.
What makes us human is our capacity for conscious thought, reason, and emotion. We see that all of these are completely dependent on brain activity. Why would this dependency cease upon death? Why is it so difficult to grasp that we are mortal, biological creatures? My theory is that believers do not like the idea of dying forever. It makes them feel uncomfortable, and they shy away from the idea. Believers are more concerned with how their beliefs make them feel, and atheists are more concerned with whether or not their beliefs are true.