Why do some people not say thank you when you hold the door for them? It is insanely irritating. 

If someone goes out of their way to perform a kind act and hold the door open, the easiest thing you can do is give them a quick thanks to show your appreciation. It’s not like you have to go out of your way to do anything, but the person holding the door for you is.

It is very easy to show gratitude for this, and the person holding the door will appreciate it.