for the same reasons that an "involuntary" orgasm "often" feels so good!
and both are much much better if you attempt not to blink during the process!
sneezing is the reflex that is closest/most similar to the orgasm - to improve it, try and hold it off for as long as possible, but if it goes away, you are going to be terribly disappointed!
it does contract/release many of the same muscles thus stimulating receptors in those areas