“Brandschutztür” is a fire proof door. These are supposed to be closed at all times to prevent smoke from a localized fire from filling an entire building. Keeping smoke out of, e.g., staircases is vital in ensuring a safe evacuation of the building.

You can often find fire extinguishers near the main access door for a floor. When people need to repeatedly move through that door, they’ll often use the fire extinguisher to (illegally) prop the door open.

The fire extinguisher asks, “What is my purpose?” Instead of answering ‘extinguish fire’ or something, Rick answers “to keep the door open”. Not only is that not the fire extinguisher’s intended purpose, it is in stark contrast to its job. Both the fire door and the fire extinguisher are meant to help in case of a fire. But the extinguisher is used in a way that puts people in danger. Hence the fire extinguisher can only mutter, “Oh god”