I'll have a go even though I'm an atheist, I've had this conversation before.
From what I know It's because their belief in god is so strong the idea that god exists becomes a fact rather than a simple belief. So because it's so obvious to them and it makes them feel great about themselves. Therefore they wonder how you can live without such bliss.
Then there's the idea of hell and the lack of belief rubbish. Also the concept of an afterlife tends to give them a point or a purpose because life is a 'test' to them.
long story short "god makes me happy why doesn't he make you happy? oh and you might go to hell if don't
"
I'll probably get criticised by someone but that's what I've heard and even partially felt when I was religious.