Newborns n sleeping on there tummy!!???

Holly
I would like to understand why they tell u not to but your baby to sleep on there stomach I mean I know they say SIDS but my son is 3 weeks old n he loves sleeping on his tummy he even gets 3-4 hours of sleep when he's on his tummy any one els out there going theu the Same thing??? I mean I worry about it but then I think my son if sleeping fine should I continue to let him sleep like that???