Words create emotions, which in turn influence behaviour. Let’s say you want to be more productive. Telling yourself I can’t watch TV all day long” makes you feel like you aren’t allowed to do what you really want. On the other hand, “I don’t watch TV all day long” frames it as part of who you want to be.

More:

I Have to Do It or I Want to Do It? (article, psychologytoday.com) by Timothy A. Pychyl

Why you should say “I don’t” instead of “I can’t.” (video) by Daniel Pink