Recent news reports have shown new signs going up on buses in our nation's capitol. They say:
"Why believe in god...
Just be good for goodness sake!"
These signs are to help agnostics and atheists feel less "left out" this holiday season.
My response to that can be found by watching this video: http://www.rhchurch.org/pages/cardboard-testimonies/
I think it answers the question, "Why believe in God?" quite well.