It's interesting that the neuroprotective chemicals also happen to result in a pleasant feeling, as there would have been no evolutionary pressure whatsoever to make a physiological state that's almost exclusively experienced just before death feel like anything in particular. Anything from bliss to agonizing pain to nothing would have been equally viable.
In fact there may be an evolutionary advantage in doing the opposite - flooding the system with stress/fear chemicals at the time of death. Predators would likely prefer to avoid tense, stringy meat reeking of primal panic and feces.
Could it be possible that the brain sensing no more blood is being supplied then triggers a mechanism to release this chemical (and in what quantities)? Wildly speculating as a layman, however, maybe there is a link between dream states / DMT production/synthesis and these experiences which are yet to be explored further.