Why do most people become angry and deny they have double standards?

For example, if you have siblings we all have one sibling that is the favorite and receives the least amount of criticism and more praise from their parents and when the other children point it out, then all of a sudden the parent denies playing favorites and having a double standard. Or when it comes to politics, if one party does something wrong then their opponents will point it out while at the same time ignoring the same corruption in their party.

Why do most people become angry and deny they have double standards?
Post Opinion