If we know something is wrong then why is it that we do it anyway?

Guilty pleasures dear, guilty pleasures.....

like when your sibling ask for your ice cream but you cant cos you cant part with even a spoonful of it even when you know sharing is caring and even when they share their everything with you....so yep....guilty pleasures

It's fun!

Because, your subconscious works so much faster then you, cognitively working things out . . . The pull to make decisions impulsively often puts actions into play far more quickly. To the point of even making micro choices that would bring you back to a pattern you either like or do not like about yourself. 

In short, your brain knows what feeds it the really nice Chemicals, versus what feeds it the ones it doesn't like . . . It will impulsively be trying to work to get the more pleasurable results . . . Even if you think you have decided against it . . .

Also, isn't chocolate delisicous?

Lack of will power

I wouldn't know. I've never done anything wrong. I'm perfect, just ask my mother.