For a time as a child, you're essentially powerless and beheld to your parents whims and feelings, some of which are not all that charitable.
For example. Your parents are unhappy with the service at a restaurant. They yell at the waitstaff and then demand to speak to a manager. You're forced to sit there while they continue to yell, yell, yell. Yet you can do absolutely nothing.
When we are little, we totally swear that we won't make the same "mistakes" our parents made raising us.
But then the time comes to be parents ourselves...and it's not as easy as we thought it was in the first place. Nothing gives you sympathy for your own parents quite like having to become a parent in your own right.
Which is why sometimes, it's hard to keep those promises we made to ourselves.