B. The Halachic obligation:[1]

One is obligated to show gratitude to every living being that caused him good, even to animals, vegetation, and even to the inanimate.

Biblical or Rabbinical: The concept of showing gratitude to an individual who provided one with good, is Biblically based.[2] Furthermore, it is considered a Biblical obligation.[3]

Similar to a sale and business transaction:[4] Doing an act of gratitude for a person whom one is indebted to is Halachically considered similar to a sale transaction. It is for this reason, that it is permitted for one to free a Canaanite slave in gratitude for a certain good that he did for him, despite the general prohibition against emancipating a slave, as this is viewed similar to a sale. [This once again emphasizes the obligatory status of gratitude.]

How to express the gratitude: At the very least, one should express his thanks to the person and acknowledge the gratitude he has towards him.[5] However, ideally, one should pay back the person whom he is indebted to even more than the value of that which he received from him.[6]

Not to pay evil for good: All the more so, is one obligated to abstain from shaming or causing evil to such a person that one is indebted to.[7] On this the saying goes, “Don’t throw a stick into the well that you drank from.[8]” Likewise, on this it states in Mishleiy[9], that one who returns evil under good, evil will not escape his home, heaven forbid.

[1] See Ramban on  Ki Seitzei 23:4-8 “They were obligated to do a favor for the Jewish people and instead they paid the Jewish people back with evil”; All sources brought below regarding showing gratitude from which it is evident that is an obligation. So is evident from the fact that Moshe refused a command from God on this basis.

[2] So is evident from all of the verses in Scripture and their commentary in Midrash brought below in D.

[3] So is evident from the Midrash from the fact that Moshe pushed back against God’s command to him to fight against Midyan, and to smote the Nile River and to confront Pharaoh, on the basis that he must show gratitude to them.

[4] See Rashba Gittin 38b

[5] See Brachos 46a; 58a

[6] See Bava Metzia 86b; Brachos 20b

[7] Avoda Zara 5a; Meiri, brought in Shita Mekibetzes Bava Kama 92b

[8] Bava Kama 92b and Rashi there, based on the allowance for the third-generation Edomite to enter the Jewish people, but not the Moabite or Amonite even in the 10th generation

[9] Mishleiy 17