Because you suck at passwords

Take a deep breath, because most of what you've been told about safe passwords is incorrect. Observing the rules might result in you creating a password that resists typical cracking techniques. But you could also be devising one that could be cracked in not much more time than “password” or “123456”. And even if your password is good, the possibility of entering it at a fake site that resembles the one you intended to use is real.

Everything You Know About Passwords Is Wrong | Fast Company | Business + Innovation

The excellent Glenn Fleishman on the good and bad in password creation. Of course as Glenn points out, there are easier and better solutions than creating your own secure passwords (cough 1Password).