Cantankerousness

Why is it that when people tell you what to do, you'll not do it just because you are stubborn or whatever. Like flossing my teeth. I've never read anything that says it's a bad thing. It's not all that hard or time-consuming for me. I have dental floss in my bathroom. I just went to the dentist so I have tons of it actually. What is the problem? My problem is that my sister is always after me to do it. So I don't want to. What kind of weird things does your brain do to you? What does it matter what she wants? Now, the dentist advised I floss and he seems nice and genuinely concerned so now I will try. What things don't you do that you should do but don't because someone is nagging you about it?