Sofa Funs v1: What is so good about girl's anal button? (Patreon)

First version of sofa funs are uploaded to early 5$+ scenes post (swf, exe, apk)

I don't usually post much gif animations for scenes I'm working over, but this one is driving me crazy and I have to share it. With just one question. 

What is so good about girl's anal button that makes you want to touch it and why so many girls hate it when you touch it?