PASSWORD REUSE
Strong is not the same as safe.
Consider a long, complicated password. It may be difficult to guess and satisfy every complexity rule you have ever been given.
Now use it on six different websites. The problem is no longer the password itself. The problem is that six unrelated systems are now connected by one secret. If any one of them exposes it, the others become targets.
ONE SECRET / MANY ACCOUNTS
River!Falcon92Mountain
EmailSAME PASSWORD
WorkSAME PASSWORD
ShoppingSAME PASSWORD