Let's do the math:

Let's say every person on earth has used ~1000 passwords so far, that makes approximately

  10 trillion passwords, which is ~2^43 If i am not mistaken. Choosing any existing password at random thus is about as good as a truly random 8-9 case sensitive character password. Not very good. See this answer https://security.stackexchange.com/a/43684/35094

That basically means that, in theory, not only should one not reuse a password, one should not reuse a password that has been used by anyone ever. Passwords that have been used before are basically one big dictionary attack waiting to happen.

Let's do the math:

Let's say every person on earth has used ~1000 passwords so far. That makes approximately 10 trillion passwords, which is ~2⁴³ if I am not mistaken. Choosing any existing password at random is thus about as good as a truly random 8-9 character case-sensitive character password. Not very good. See this answer.

That basically means that, in theory, not only should one not reuse a password, one should not reuse a password that has been used by anyone ever. Passwords that have been used before are basically one big dictionary attack waiting to happen.