The chachomim instituted that we should read a haftorah every Shabbos that deals with a topic which is inyonei d'yoma. Usually the haftorah will relate to some theme which appears in the parshas ha'shavua, as we consider that to be inyonei d'yoma. When Rosh Chodesh fall out on Sunday, we consider Mochor Chodesh to be inyonei d'yoma.
The second pasuk in the haftorah of parshas Eikev says, "gam eileh tishkachna v'anochi lo ehskocheich - this happens sometimes as well, but I (Hakadosh Baruch Hu) will not forget you (Bnei Yisroel)." The Medrash sees an additional level of meaning in Hashem using the words "eileh" and "anochi" when telling Bnei Yisroel what He will remember us. Bnei Yisroel listened attentively when Hashem proclaimed "anochi Hashem Elokecha", but then did not listen to Hashem when, at the time of the chet haeigel, they used the expression "Eileh elohecha Yisroel". The Medrash explains that Hashem is telling us that He will forget about the chet haeigel, but He will not forget about the zechus of ma'amad Har Sinai. How does one understand this selective memory on the part of Hashem?
The explanation is not all that complicated. In life we all have people and places that we like and those that we dislike. All people definitely have ma'alos and chesronos. We like certain people and places because we feel that the ma'alos outweigh their chesronos. And the same is true in the people and places that we dislike. We recognize that they too have ma'alos but we have decided that their chesronos outweigh their ma'alos. One cannot lead a normal life by having a love-hate relationship with everybody, loving them for their ma'alos and disliking them for their chesronos. We always have to decide whether any particular individual is worth liking and then simply ignore their chesronos; or perhaps we should decide to dislike him and them ignore their ma'alos. The Ribbono Shel Olam has decided to love Bnei Yisroel because of the Avos (see Parshas Eikev 10:15 and 9:5) and therefore to chooses to ignore all of their chesronos.
When we recite the Selichos we mention that Hashem is Ma'avir rishon rishon. The Rambam explains (Hilchos Teshuva 3:5) that what this means is the following: when one's sins are counted up to determine whether his mitzvos outweigh his aveiros or the reverse for an individual (as opposed to the tzibbur) the first two sins are discounted. If after discounting the first two times it turns out that his mitzvos outweigh his aveiros, all of the aveiros will be discounted because after discounting the first two aveiros, the third and the fourth are now considered as the first and the second and deserve to be discounted. Then, after they too are discounted, the fifth and the sixth are now considered as the first and the second and therefore deserve to be discounted, and so on. This concept probably only applies to Bnei Yisroel as they are basically, at their very core, full of ahavas Hashem and yiras Hashem in their spiritual DNA. Rebelling against Hashem and doing aveiros simply does not fit in with their personality at all. The aveiros that they do (when outnumbered by the mitzvos) are considered an anomaly and a quirk. It is for this reason that Hashem will forget the eileh and only remember the anoichi.