One of the many reasons to NOT use the same password is the onslaught of database breaches that seem to be occurring at an exponential rate. If you use the same password on multiple sites that one breach email and password could then be used to gain access to many other sites.
I know this first hand as I have been the victim of many breaches, credit card thefts and even an identity theft. Fortunately I have learned how to mitigate the severity of these security issues.
There is a great website that I use and highly recommend called haveibeenpwned.com Pwned loosely means owned and in this case stolen credentials. You can plug in your email address for example and it will cross reference your email to their databases of various breaches. The site is secure and does not spam you when you utilize their services. You can also search by password or domain name.
I suggest the use of a password manager to create and keep track of all your passwords. Satuit4me provides various security services to help limit your risk.