By default root password is your user password, I guess that the reason is that its a much more secure approach when the password will be the one that is set by you (assuming its a good - difficult to hack password) rather than some well known password like "root" that is always in place by default and known over the Internet (security risk).
There is a good discussion about the topic on linuxmint.com forum - citing from there user karlchen:
During the Linux mint installation, you create your first user account. You assing a password to this first user. The installer silently assigns the same password to the user account root. That is does do so can be found in the Official Linux Mint User Guide as well. (cf. e.g. p. 20) So if you remember your initial user password, you know your root password as well. In case you change your user password later on, doing so will not change the root password.
Source - page 20 last paragraph: https://www.linuxmint.com/documentation/user-guide/Cinnamon/english_18.0.pdfhttps://www.linuxmint.com/documentation/user-guide/Cinnamon/english_18.0.pdf