I've never been good at 'blessing' people when they sneeze.
The natural response just doesn't come naturally.
When people say it to me, all i can say is thanks - but what am i thanking?
So, i looked it up and here's the low down:
It was either
1) Used as an effort to fight against disease especially around the time of the bubonic plague
2) When you sneeze your heart stops beating so was used to ensure return of life by encouraging your heart to start beating once again
3) Sneezing opened your body to invasion by the devil so was used as a shield against evil
Bottom line is there's no definite origin for the custom... so now it's just habit i guess.
I think i don't say it because firstly 'bless you' is a religious term and i'm atheist, and secondly i forget.
For me sneezing is the just same as coughing... and people never say 'bless you' when you cough.