Reusing a password turns one compromised account into a risk for other accounts. When login details are exposed in a breach, criminals may try the same email-and-password combination on unrelated services. Unique passwords limit that spillover because a password stolen from one site does not unlock the others.
A password can be strong in isolation and still create a serious problem if it is reused. The weakness is not necessarily the password itself; it is the connection between accounts.
One breach can travel
Websites sometimes suffer data breaches in which account information is exposed. Attackers know that many people reuse credentials, so stolen username-and-password combinations can be tested against other services. This pattern is commonly called credential stuffing. If the same password protects email, social media and shopping accounts, a compromise at one service can create a path into several others.
Email is especially important because many services use it for password resets and security notifications. Losing control of an email account can therefore make other accounts harder to protect.
Unique passwords contain the damage
Using a different password for every account changes the situation. A breach may still expose one credential, but that credential should fail elsewhere. CISA and NIST both recommend strong, unique passwords and encourage the use of password managers, which can generate and store long random passwords so users do not have to memorize each one.
Multi-factor authentication adds another barrier. A stolen password alone may not be enough when an account also requires a second factor. Passkeys can go further by replacing the traditional shared password with cryptographic credentials designed to resist common phishing and reuse problems.
The goal is not perfect secrecy
No user can guarantee that every service they use will never be breached. Good account security is therefore partly about limiting consequences. Unique credentials, multi-factor authentication and careful recovery settings make it harder for one incident to spread.
The practical rule is simple: do not let the security of many accounts depend on the same secret. Reusing a password creates a shared failure point. Unique passwords or passkeys turn separate accounts back into separate security boundaries.