Why should we avoid reusing passwords?
The short answer
If you reuse one password, a breach at one service can put every account using that password at risk. Unique credentials limit the damage to one service when possible.
How the risk spreads
Attackers may obtain a leaked email-and-password pair and automatically try it on other sites. Even a strong password becomes a weak point when it is shared across services.
What can help?
A reputable password manager can create and store long, hard-to-guess, unique passwords. Enable multi-factor authentication where available, and change credentials that may have been exposed.