Near Death Experiences - My rational take on it (and not my own thought here but this is based on what I know and have read, so somewhat of a rehashing of already known facts):
DMT - Dimethyltryptamine
This substance is produced by the human pineal gland in the brain. It's what makes you dream, and at death the pineal gland releases all of it's DMT. This causes you to have very intense very real (after all, what is real? Real is simply our brain processing information coming in from our 5 senses right? That is as real as things can get for a human) visions which seem like reality. Just as a very intense realistic dream may be very hard to forget once you awaken from. It may seem "real" but it's simply a process of the brain and DMT.
So, when a person get's very close to death as in a NDE the pineal gland does it's natural function of expelling all it's DMT and a NDE happens. It seems as real as real can be for a human and thus when the person is "brought back" to life he/she remembers this vision and attributes reality to it, viewing it as "real" as in, "This is the truth! This is what happens at death! I have experienced it!" etc......
These NDE are all different. If you look at them all, explore them, you will find they vary greatly. IF they were reality would they not all be the same? The world around us is the same for everyone. We breath air, we eat food, we walk on the ground, etc.... it's in this way I'm speaking of it all being the "same". NDE are very different for the different people who construct them. No basis in reality, simply a function of DMT and their own individual brains.
In short, NDE are nothing more than a very real feeling complex dream caused by the release of DMT at the time of death.