I "shush" therefore, I am.
Reasons for which we "shush":
1. It's easier than going after each patron and asking them each individually to keep it down. We'd be following everyone around.
2. Because it's more effective than having to explaing to patrons, "You are being very loud, can you lower your voice". It pretty much gets the point across.
3. Because when people are walking/running into the library and talking so loudly they are practically screaming at each other, it's the only way we can stop ourselves from screaming back, "Hey, Shut the F up! You're in a library for heavens sake. Show some respect!"