Suspicious secure log in log

I just created my digitalocean account a few days ago, and I just found that there are a very big number of Root log in fail logged in my /var/log/secure. It shows 67374 lines in that file. For instance, I only logged in once today but it shows lots of logs in. The login attempts are performed by my own user. These are a few lines: 67366 Jul 3 22:33:59 lingvi sshd[28849]: Failed password for root from 43.255.188.145 port 55580 ssh2 67367 Jul 3 22:33:59 lingvi sshd[28849]: Received disconnect from 43.255.188.145: 11: [preauth] 67368 Jul 3 22:33:59 lingvi sshd[28849]: PAM 2 more authentication failures; logname= uid=0 euid=0 tty=ssh ruser= rhost=43.255.188.145 user=root 67369 Jul 3 22:33:59 lingvi sshd[28851]: pam_unix(sshd:auth): authentication failure; logname= uid=0 euid=0 tty=ssh ruser= rhost=43.255.188.145 user=root 67370 Jul 3 22:33:59 lingvi sshd[28851]: pam_succeed_if(sshd:auth): requirement “uid >= 1000” not met by user “root” 67371 Jul 3 22:34:01 lingvi sshd[28851]: Failed password for root from 43.255.188.145 port 42254 ssh2 67372 Jul 3 22:34:01 lingvi sshd[28851]: pam_succeed_if(sshd:auth): requirement “uid >= 1000” not met by user “root” 67373 Jul 3 22:34:03 lingvi sshd[28851]: Failed password for root from 43.255.188.145 port 42254 ssh2 67374 Jul 3 22:34:04 lingvi sshd[28851]: pam_succeed_if(sshd:auth): requirement “uid >= 1000” not met by user “root” :set number

The logs are constantly increasing in an automated way. What could cause this?