F. Hakaras Hatov to parents:[1]

The root of the command of Kibbid Av Vaeim is due to that it is befitting of a person to recognize and act with kindness towards one who has been kind to him. One should not be ungrateful and ignore [one who has done for them so much good] as this is a most evil and repugnant trait before G-d and people. One is to recognize the fact that his father and mother are the ones who are responsible for bringing him into the world and accordingly it is fit that he gives them all the honor and assistance that they are capable of. One must also recognize that his parents put much effort into him when he was young [in order to educate him and support him] and they nursed him and fed him and dressed him.[2]

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[1] Chinuch Mitzvah 33 “The root of this command is due to that it is befitting of a person to recognize and act with kindness towards one who has been kind to him. One should not be ungrateful and ignore as this is a most evil and repugnant trait before G-d and people. One is to recognize the fact that his father and mother are the ones who are responsible for bringing him into the world and accordingly it is fit that he gives them all the honor and assistance that they are capable of. One must also recognize that his parents put much effort into him when he was young. When a person has this form of recognition and gratefulness to his parents, it will lead for him to also develop this recognition and gratefulness towards G-d who is the cause of the existence of his parents, up until the first man and is the one responsible for bringing him into the world and giving him all his needs, and health and sanity, of which without it he would be like a horse and mule who have no knowledge. A person will then conclude from this how much he must be careful in the service of G-d.”; Yerushalmi Peiah 1:1 and Kiddushin 1:7; Chareidim 9:37; Keli Yakar Yisro 20:12; Even Ezra 20:1

[2] Chinuch ibid; Even Ezra ibid; See Likkutei Sichos ibid p. 91 as to why Ramban 20:12-13 omits this point.