Why does my cat let me kiss her tummy willingly?

My pet Coco who is a 4 year old 1/2 russian blue 1/2 bermese breed cat will roll on her back in the star fish position waiting for me to kiss or pet her tummy she has NEVER bitten me nor scratched me EVER. she just lays there she is a beautiful personality so my question is... do all cats do this with their owners or is it rare? she also snuggles between my feet when i sleep under the covers...