You went through a lot of trouble to come up with really strong, unusual passwords - maybe even two or three. It took time to memorize them. Why not just use these same ones over and over in rotation?
Because that greatly increases the chances that, once someone steals or figures out one of those passwords, they can gain access to not just one but many accounts - and commit identity theft.
One of the ways security breaches happen is when old account data isn't archived securely. If you are still using (or reusing) a password from years ago, then you are vulnerable.
Of course, the more devices and accounts we have over time, the harder it is to keep track of passwords. Using a password-tracker app that encrypts passwords can help.
Don't share your passwords with other people.
Create strong passwords. Stronger ones are unusual phrases that combine capital letters and lowercase, incorporating numbers and special characters (at least 8 characters long but more is better).
Use different passwords for each login. It is so very tempting to reuse passwords, but this is the easiest way for a hacker to get into your accounts once there has been a security breach.
Change passwords often.
Don't include personal information (like phone number or birthday) in your password
Use two-factor authentication, security questions, or biometric data (like fingerprint or face recognition) if available.
Better yet, use a password-encypting app to create and remember all your passwords. (Here's an app review from Wired Magazine that includes free apps.)