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

Sneezing has probably evolved relatively recently. It seems to involve cerebral processes (often it can be suppressed and also one does not sneeze while sleeping - one wakes up just before. However the actual process once initiated uses primitive parts of the brain (similar to hiccups) and coopted is the totally involuntary blink.

Play with bodies, the ultimate educational toys. And in physiology exams they can't stop you bringing it in to use as a cheat sheet.

IF YOU TILT YOUR HEAD BACKWARDS EXPOSING YOUR NECK, IT IS POSSIBLE TO STIFLE YOUR SNEEZE WHICH CAN BE USEFUL AND GRATIFYING.  HOWEVER, THE SNEEZING FUNCTION ALLOWS ONE TO CLEAR PARTICULATES FROM THE UPPER RESPIRATORY TRACT.  THIS IS BEST DONE WITH A TISSUE OR HAND OVER THE NOSE AND MOUTH IN CONSIDERATION TO YOURSELF AND OTHERS WHILE WE ARE ALL INHALING AND EXHALING.