Answers:
a sneeze is an autonomic response to irritants in the nasal mucosa.
See RN's answer
presence of noxious substances at the mucous membrane of nasal cavity produce irritation to the sensory bottle ending of the trigeminal effrontery influential to reflex sneezing to expel the noxious substance.
our body is trying to achieve germs out.
I give attention to it is personality's opening of getting race to relate you gesundheit..lol
No, seriously it is to blast venomous vermin out of your muzzle.
Or as my grandson like to influence, "Because I don't know why"
Here is some actual information
http://en.wikipedia.org/wiki/sneeze...
Plenty more if you google <why sneeze>