Why is it dangerous to use the same password

Question : 123456, password or qwerty have something in common: what is it? 

Answer: They are among the weakest words internet users are using. 

Worse: according to a study, 93% of French Internet users use weak words when 60% use the same password.

If you always use the same password or passwords that are close to each other, then as soon as this password is discovered by someone, they will be able to access all your different services.
All it takes is for someone with malicious intent to see you give it to him, or to be able to access the database of one of the sites you use.

On the other hand, if your passwords are all different, then the discovery of a password is less serious: only the data of the site in question will be corrupted.
It is particularly important to use a unique password for its most sensitive services such as access to its e-mails or its bank.