You walk across the rug, reach for the doorknob and..........ZAP!!! You get a shock.
Why does it happen in the winter? It is because of static electricity. And it is the nature of matter.

Why explaining above? Because we want to explain that it is not a doorknob's fault. As you walk across a carpet, electrons move from the rug to you. Now you have extra electrons. Touch a door knob and ZAP! The door knob is a conductor. The electrons move from you to the knob. You get a shock. We usually notice static electricity in the winter when the air is very dry.

It is just same as you get a shock when you touch your earphone.
Please touch any metal parts around you to remove electrons from your body in advance using the earphone.