That's really not a good habit to get into at such a young age. I'm not much interested in going into the 'whys' of it all so let's just leave it at this murky level and revisit it at a later time, if necessary.
As I listen to myself talk to my children on a daily basis, I have come to the conclusion that I use the word 'Don't' more than any other word in the English language. I actually try not to; I try and make an effort to focus on what they can do or should do, but sometimes (okay, many times) I resort to a quick "Don't...whatever" because it seems more concise and effective. Whether that's accurate or not, it's true. And so begins the series of 'Don'ts...'