Chovos Halevovos Shaar Hebitachon. Over there he says that if you do something for someone, one should not expect a thank you because you didnt do anything for them. Hashem is the one who did it. Meaning, if you do me a favor, it is because Hashem said that I need a favor done to me and you will be the shaliach. Mitzad mentshlichkiet I should and am obligated to thank you, but in truth who I must really thank is Hashem. Does anyone remember learning about Avraham Avinu? He used to invite people and give them food and drink…Hachnasos Orchim. Afterwards the people were apprciative and wanted to thank him. Wat did Avraham say? Dont thank me thank Hashem. Now we are inclined to think wow Avraham is such a tzaddik he doesnt want to take credit…The reality is, we have no right to take credit for doing something for someone. Avraham Avinu is telling us that the thank you, ALL THANK YOUS belong to Hashem.
So if you do someone a favor and they dont thank you, they arent a mentsch but you have no right to be upset and insulted and furthermore, you must continue to do favors for people even if they dont thank you and even if they dont appreciate you!
Im sorry if this sounds radical but you can find this concept in the beginning of shhar habitachon in the chovos halevovos.
EDITED