Why do you do so?
Personally, if I hold a door or let someone through and they don't acknowledge and thank me, I don't let it bother me and simply get on with my day. The reason for this is I quite honestly couldn't care less how a total stranger behaves towards me, within reason of course, because they are exactly that. A stranger that means nothing to me.
Therefore, if they don't say thank you, it doesn't matter enough for me to need to react to it, but for many others, they have a different mindset. Help me understand! Why is this?
Surely someone would need to be important to you for their behaviour to affect you enough to react to it?