Parshas Ki Savo begins with the mitzvah of bikurim, which has two parts. First, bikurim is one of the twenty-four matanos which we give to the Kohanim. Shevet Levi did not get a share in Eretz Yisroel because the Kohanim and le'vi’im are supposed to take care of the Beis Ha'mikdash, so we’re commanded to give the Kohanim twenty-four matanos. Secondly, when the farmers bring bikurim to the Beis Ha'mikdash, they recite “Arami oveid avi”. Rashi, in his commentary on that possuk, quotes Sifrei’s explanation that the idea behind mikra bikurim is to express our thanks to Hashem for the bountiful year. In another two places in Chumash Devarim, the Torah instructs us that after being blessed with a successful in parnassa, we should not forget that the source of all the parnassa is Hashem. In Parshas Va'eschanan (6:12) the Torah warns us not to forget that after receiving a successful parnassa, we should be careful not to forget about Hashem. The same idea is repeated in Parshas Eikev (8:14), i.e. that we should not become arrogant and forget about Hashem.
One of the ba'alei ha'Tosfos, Rabbi Moshe from Kutzi, wrote the sefer known as Sefer Mitzvos Gadol (a.k.a. Smag). After completing the entire sefer he had a dream that one of the mitzvos in the Torah came to him and complained why it was left out from the count of the mitzvos in the sefer. The next day, the Smag reviewed the Chumash again and realized that he left out this mitzvah that appears in Va'eschanan, Eikev, and in Parshas Shoftim (18:20) that the king has to have a small sefer Torah with him at all times which he must use to learn every day and every night in order that his position should not go to his head and he not become arrogant and forget all about Hashem. (This mitzvah #64 of the lavim is based on a Gemarah in the beginning of Sotah).
Hashem is invisible so it is very easy to forget about him. The mitzvah of “Anochi Hashem Elokecha” does not mean that we should believe in the existence of Hashem, because if He does not exist, then there is no one there to give us any commandments! (As the Ramban points out in his commentary on the Rambam's Sefer Ha'mitzvos). Rather, the nature of the mitzvah of Anochi really is, as the possuk reads in Tehillim, “Shivisi Hashem l'negdi tamid”, i.e. to keep in the forefront of our minds that Hashem is always present, and we must act and speak accordingly. Based on this mitzvah, we should have the attitude that Hashem is at the center of everything that is happening in the world. Arrogant people who think highly of themselves always visualize as if they are in the center of everything that is happening in the world and that everything revolves around them. That is why the Smag formulated this new mitzvah that one should not be arrogant in order that he should not forget Hashem. The Mishna (Pirkei Avos, chapter 3) teaches us that when we give tzedakah, we should not have the attitude that this money is my hard-earned money, and I have the right to distribute my charity wherever I please. “Tain lo mi shelo” - we should view it as if we are merely an apitropis (guardian) on Hashem's money, because Hashem is the one who gave us the chochmah to earn all the money and, as we say in the Selichos prayers, “ha'neshoma lach v'haguf shelach”; our bodies belong to Hashem and the chochmah that we have is all from Hashem. Therefore all of our earnings really belong to Hashem.
The Gemarah (Baba Kama 92b) records a popular expression: when one goes to a wedding the wine is paid for by the baalei simcha, the waiters are salaried to serve the wine, and the waiters are tipped in addition, and still, we say thank you to the waiter when he pours us some wine. The Chachomim felt that it is very important to always express gratitude to others. One can always explain away why there is no need to thank others for anything they do for us, since there is always reciprocity in human relations - I do for others in exchange for them doing for me. And yet, the chachomim felt that because Hashem is invisible and we can easily forget about him, it is important that we always insist on ha’koras ha'tov towards other human beings to make sure that we don't forget about Hashem. The Gemarah (end of Berachos) discusses the text of the bracha of “nishmas kol chai”, which is really the beginning of Yishtabach. There are two girsa’os in the text of the Gemarah, the one followed by Ashkenazim and one followed by Sefardim. In nusach Ashkenaz, we recite the text which says that Hashem is, “mehulal b'rov ha’tishbachos - praised by us with a majority of the praises”. The nusach Sefard girsa in the Gemarah comes to the conclusion that we ought not say that expression because it implies that Hashem does not deserve all of the praises, only the majority of the praises. The reading in the text of the Gemarah that the chachmei Ashkenaz had was that because of this objection we mention both expressions: “Melech mehulal b'tishbachos - the King who is praised with all of the praises”, as well as “ha'mehulal b'rov ha’tishbachos”. Rav Soloveitchik suggested that the nusach Ashkenaz may be teaching us that perhaps it is not such a smart idea to reserve all the praises for Hashem. Because He is invisible, He can easily be forgotten about. Therefore, we should get used to expressing ha’koras ha'tov to other human beings, even if they really don't deserve it due to there being reciprocity anyway, because we must strengthen this important middah of ha’koras ha'tov, so that we will have proper ha’koras ha’tov vis-a-vis Hashem which, in turn, strengthens our emunah in Him, specifically our recognition that He is the center of everything in the world.