Why is it that people adjust rules for their own convenience?

Everybody has rules in their homes that they do not like at all. In fact if you like every rule that you have in your house there's probably something wrong with you. Since we don't really like these rules, from time to time we adjust them so that we do like them. One of the rules that most teenagers tend to bend is the curfew rule set in their homes. If you happen to walk in the door five minutes after you were supposed to be home and your parents are really strict on that, then you'll be spitting out about every excuse you can come up with to avoid punishment. However, if one of your other siblings happens to walk into the house after you, you want them to get the punishment that they deserve. Why is that? Why is it that people constantly do this type of thing. Once you narrow everything down, you can usually come to the conclusion that it was for a selfish reason. It's in our human nature to get everything we want and if that requires bending the rules to get it, then so be it. It's really sad but it's true. If there weren't any rules then that would be another disaster. If we can just learn to live with the rules that are given to us then life will be a lot better.