[EvertonDiehard]

Why must you hold the door for the person behind you leh? You might be treated as a doorman lah! Must you thank the doorman leh? That's his job lah!

Any person in his right mind will know that I am not the doorman, so it is basic courtesy to say "Thank you". When the roles are reversed, I make it a point to thank the person holding the door open for me, and he/she either breaks into a smile or says "You are welcome". A basic gesture like that can make the other individual's day.