The ideal of the perfection of giving--is "no giver, no receiver.

Have you ever seen someone do something courteous and then get angry for not being properly thanked? True compassion has no expectation of reward or even a simple "thank you" attached to it. To expect a reward is to maintain the idea of a separate self and a separate other.

The ideal of the perfection of giving--is "no giver, no receiver." For this reason, by tradition, begging monks receive alms silently and do not express thanks. Of course, in the conventional world, there are givers and receivers, but it's important to remember that the act of giving is not possible without receiving. Thus, givers and receivers create each other, and one is not superior to the other.

That said, feeling and expressing gratitude can be a tool for chipping away at our selfishness, so unless you are a begging monk, it's certainly appropriate to say "thank you" to acts of courtesy or help.