Why do cats loves to be scratched?

You know, I've always wondered this but why do cats love to be scratched and petted so much? I can only imagine that they have sensory glans or something that are very sensitive and feel good, not too be crude but is it similar to humans having their genitals touched or something similar that is sensitive? Because a cat will literally let you pet it until your arm loses feeling and it will STILL enjoy it then.