why do we sneeze?

Here is the best explanation I found.

In layman's terms, we sneeze to rid our body of harmful things, germs, bacteria, and irritants.

There are some people who sneeze when the look at the sun or even bright lights. This is called photic sneezing, and seems to be a genetic trait. It may be some type of allergy. I have read that one-fourth to one-third of all people are photic sneezers.