If you re-use passwords, once one password has been compromised, it’s only a matter of time before criminals will be able to gain access to your other accounts and steal information for financial gain.

Discussion <<< : dont agree with this answer, maybe i dont understand "sites" if you logon you get access to many resources at many sites for me this scenario is better than to share passwords with a lot of peoples.

(Comment Added By MTG 2/16/2016.

I agree with the answer and here is why I agree. Just say a users machine is compromised by spyware and that user uses the same password for every site. Spyware can gather what sites a device is making a connection to & user web surfing habits. If the password is the same for every site and that hacker compromises the password then every site you authenticate to becomes compromised because you are using the same password for every site. Instead of just 1 site account being compromised, all sites you sign into are compromised.

[By that logic, Single Sign-On is a bad idea.]