Every server outage traces back to one of three sources. Naming them is the first step to managing them, because each one needs a different defence.
The first source is you, the operator. A mistyped rm -rf, a firewall rule that locks out SSH, a config reload with a syntax error at 2am. Across most small teams, operator mistakes cause more downtime than attackers do, because you touch the box every day and attackers do not.
The second source is attackers. Exposed services, reused passwords, an unpatched CVE. A fresh server on a public IP starts seeing automated SSH brute force attempts within minutes of coming online.
The third source is entropy: the slow drift that breaks a machine nobody touched. Disks fill with logs, a TLS certificate expires, a package upgrade changes a default, a cron job silently stops firing. Entropy is the one people forget, because there is no villain and no keystroke to blame.
Safe management is not one skill. You reduce operator error with process, attackers with hardening and patching, and entropy with monitoring. If you only cover one of the three, the other two still take you down.