By default, the SSH server denies password-based login for root. In /etc/ssh/sshd_config, if the following line exists, possibly commented out (with a # in front):

PermitRootLogin without-password

Then change it to the following, uncommenting if needed (remove the # in front):

PermitRootLogin yes

And restart SSH:

sudo service ssh restart

Or, you can use SSH keys. If you don't have one, create one using ssh-keygen (stick to the default for the key, and skip the password if you feel like it). Then do sudo -s (or whatever your preferred method of becoming root is), and add an SSH key to /root/.ssh/authorized_keys:

cat /home/user/.ssh/id_rsa.pub >> /root/.ssh/authorized_keys