“secondly since when do you have to thank someone for holding a door “
EVERY time they do this for you. Didn’t your mother teach you about good manners? I am surprised that any frum person whould say what you just did.
Popa is correct that not always does someone realize you held the door for them. But if they DO realize it, a simple thanks is warranted. Nobody is your personal doorman. It is a kindness that they held it for you. And people who do not appreciate that kindness deserve to have the door slammed in their faces. JMO.