Sulley: You don't study scaring, you just do it.
Mike Wazowski: You don't think I'm scary.
Sulley: You're not even in the same league with me.
Dean Hardscrabble: I'm here to make good scarers great, not make mediocre scarers less mediocre.
Young Mike: I found a nickel! I wish I had pockets.
Mike Wazowski: I've been waiting for this my whole life! I'm gonna be a scarer!
Answer: Well a door first needs to be activated from the monster side. See the doors of the human world lack the receiver for the energy they need to open the doorway between worlds (the red light on top). That's why in the monster world they hook a door up to a machine to activate it allowing the passage between worlds before any scream or laugh is made. It costs power to activate a door to allow a monster in. Boo was able to activate doors because she was in the monster world and her screams and laughs triggered the devices on top of the doors, activating them automatically. This was the first time ever a human entered the monster world.
lionhead