We kept our doors open majority of the time, it makes you seem more friendly. 
But it's not quite as safe to do that, anymore. They don't even leave classroom doors open. We shut them, when class starts, and, often, we lock them.
I fail to see what an 'open door' does, other than invite strangers you don't know, in. Why would you want to have your door open, where anyone could get in, and steal stuff? I Don't get it.
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*Forever & Always...Yours*