In this week's Parsha, the Jews go through an obvious change in their relationship with Hashem. This relationship really begins when the Jews are supposed to enter Eretz Yisrael, as described in Parshas Mishpatim (שמות כ"ג:כ'), where Hashem tells them that He will send a מלאך, an angel, to lead them to and through Eretz Yisrael, as described there (שם פסוק כ"א). The major characteristics of this מלאך are לפשעכם" לא ישא," "he will not forgive your transgressions," and "שמי בקרבו," "my name is within him." The "שמי בקרבו" description is basically conveying the fact that this מלאך is really Hashem Himself, while the לא ישא" לפשעכם" description conveys Hashem's warning to Bnai Yisrael that this מלאך will not take any of their nonsense, neither their complaining nor their sins, and that they therefore should not "mess with him," for if they do, the result will not be pretty.
This relationship between the Jews and Hashem built on דין, strict judgement, is truly the ultimate relationship. If one is good, life will be terrific, but if one is bad, he will be duly punished. This relationship, however, for most human beings, is too much to handle. As we see in our Parsha the Jews make a bad mistake, committing the rather serious sin involving the golden calf, the מעשה העגל (שם פרק ל"ב). For this crime, with מדת הדין, the attribute of Divine justice, lurking, death is imminent. Moshe, however, pleads with Hashem as he begs Him not to kill them (שם פסוקים ל"א,ל"ב). Hashem agrees to this, but there is one catch. Hashem is changing the nature of the מלאך that He is sending. The מלאך no longer has שמי בקרבו"," the name of Hashem within him, because Hashem has, in a certain way, left the Jewish people. This is clearly shown as well by the fact that Moshe removes the אוהל מועד, the tent representing Hashem's presence, from the people's camp (שם ל"ג:ז').
At this point, the Jews are at their lowest point in their relationship with Hashem. Moshe, meanwhile, is at his highest point, because when the אוהל מועד is outside the camp, Moshe is then able to communicate with Hashem פנים אל פנים, face to face (שם פסוק י"א). This is as close to Hashem as one can get; Moshe can get this close to Hashem specifically now, because now the sinners are far away, and there is thus nothing to block the communication between himself and Hashem. This is also very similar to what happened at Har Sinai, where the Jews were fenced in down on the bottom of the mountain. Here, however, Moshe realizes immediately that the Jews cannot survive in their current relationship with Hashem and he thus begs Hashem to come back to the camp. Hashem consents, as He comes full circle and says "נשא עון ופשע," indicating that He does indeed forgive sins (שם ל"ד:ז'), the exact opposite of the message conveyed earlier.
At this point, Moshe is ecstatic as he thinks that he can have everything Hashem can offer. He thus asks Hashem הראני נא את כבודך"," "please show me Your glory" (שם ל"ג:י"ח). Hashem's answer, though, is quite peculiar, as He says כי לא יראני האדם וחי" לא תוכל לראות את פני," "you can not see My face, for no man can see Me and live" (שם פסוק כ'). There are two problems with this response. First, it doesn't really answer the question. Second, we just saw that Moshe was talking to Hashem פנים אל פנים (שם פסוק י"א). So what does Hashem mean here? We can answer this by saying that at this point, Moshe was really asking Hashem whether now that He had promised to come back into the camp, he (Moshe) would still be able to see Hashem's face. Moshe was thus really asking to have everything: for Hashem to be within the people, but for himself to still be able to see Hashem's face. But Hashem responds in the negative, saying that if He comes back to the camp, Moshe will have to be more like a regular person. Hashem (שם) thus calls Moshe an ordinary אדם, a man, and Moshe has made the supreme sacrifice, being ready to go down a level in his relationship with Hashem in order to bring the Jews a step closer in their relationship with Hashem.