C. It’s importance:

Having gratitude for those who have done good for oneself is a most important attribute which has been praised by our sages.[1] Whoever does not have gratitude towards those who do for him good, will in the end also come to denying the good that G-d has done for him and come to deny G-d.[2]

A blemished soul and un-Jewish character trait:[3] Not having gratitude for one who has provided him with a kindness creates a great blemish within the soul of the individual. So is evident from the fact that G-d prohibited the nation of Moav and Amon from marrying into the Jewish people even after they convert, and even after many generations, because of their ingratitude to the Jewish people, as they were obligated to provide the Jewish people with kindness during their travel to Israel in exchange for the kindness that Avraham did for their ancestor Lot.

A most repugnant trait:[4] The root of the command of Kibud 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 the good that he received, as this is a most evil and repugnant trait before G-d and people.

[1] See Pesachim 118b; Bava Kama 92b; Tana Dvei Eliyahu Raba 18

[2] See Yerushalmi Brachos 9; Rabbeinu Bechayeh Shemos 1:8; Tanchuma Shemos 5

[3] See Ramban Devarim 23:8; Sheim Derech Devarim Mamarim Mamar 10

[4] Chinuch Mitzvah 33