Explain yourself and your reasons

Because I said so!

How many times have you heard or said that?

I say it when there is no major reason for a situation to happen, it just has to happen, I just want it done, no questions asked.

However, I do like to explain my reasoning to my children, when I can, so they can understand the principles behind choices.

For example, 

I tell my children I'm feeling tired and emotional and need space instead of just yelling at them because they're in my space.

Apologise if you did yell at them and explain that you felt tired and took your mood out on them. Though make sure it's not a re-occurrence. Nothing worse than someone saying sorry only for them to repeat it again and again and again. Are they really sorry if they keep doing it?

I tell my children they're too young to be doing certain activities instead of just telling them they can't do something.

I give them the knowledge to understand there is method to the goings on in every day life.

Because I said so has it's time and place, yet it's not the answer to everything and doesn't teach children the reason behind an adult's decision.

Talk to your children, explain your reasoning.  This is how they learn how to reason and it's how they also learn to parent well.