As a parent, I often found that if I actually explained why instead of the usual "Because I told you so", then I got a lot further in making them rationally arrive at the right behavior themselves (as toddlers are wont to do). I suspect that the "I told you so", not only does it completely nullify their desire but it also forces them to accept not learning and hurts their pride (which is where the tantrum comes from). These are undesirable outcomes and since parents use this trick all the time, it leads to learned behavior. Disclaimer: This is just my own analysis and I know there are times when it's too challenging to do this but it's a principle you have to focus on.