The reason for the bulletproof locked cockpit door is to keep the flight attendants from storming the cockpit and attempting to issue orders, like these three flight attendants thought they'd try on one of my flights:
I was out on the tarmac having a smoke at the time:
This is how it's supposed to look. The First Class section is even nicer:
That's why I get paid the big bucks - to exercise overall command of the aircraft and maintain control of the flight deck at all times. Some days work out better than others.