Excerpt of logic:
For most websites the username and password is all that protects your account. Other than not using an incredibly obvious password, the most important thing is to not use the same username and password combination on different websites. The more websites you use, the more likely it is that logins will get leaked, or someone will decide to sell them to someone. The most important account you have is your email account. From there someone could easily take over your accounts on most websites. Of course then you should never ever use your email password on any other website. So in summary: Use different passwords on the websites you use.