עשיתם לכם עגל מסכה סרתם מהר מן הדרך אשר צוה ה' אתכם וגו' (ט-טז)
The gemara (עירובין מא:) states: “Three things deprive a man of his senses and of a knowledge of his Creator: Idol worship, an evil spirit and oppressive poverty.” It is extremely difficult to be in control of one’s senses and be fully cognizant of Hakadosh Boruch Hu when one is plagued by the above three evils.
A "רוח רעה" - “evil spirit,” according to the Ramba”m, is any injury that does not come from man, whatever its cause may be. According to the Seforno, an “evil spirit” is any illness which causes man to sin against his Maker.
On the other hand, "דקדוקי עניות" - “Oppressive poverty,” can only be overcome with great difficulty. In fact, says the Maharsha, it is even easier to subdue one’s Yezter Hara - Evil Impulse, then to conquer such poverty.
R’ Yaakov Kamenetsky ZT”L illustrates the dire consequences of “oppressive poverty” as related to the Golden Calf incident. In reality, says R’ Yaakov, we are unable to put ourselves in their place. But imagine for a moment, such a large multitude of people - 600,000 strong - standing in a desolate desert without any sustenance of bread or water. Why should we wonder at their inability to withstand this great trial? They all knew that the Mann descended for them only in the merit of Moshe, and when it appeared (through an optical illusion engineered by the Satan) that Moshe was dead, they thought it would no longer descend; there would be no more food for their children. The thought of being stranded in the harsh desert with no food and no apparent means of support was terrifying. Indeed, this was the inception of “oppressive poverty” - a mindset which deprives a man of his senses and a knowledge of his Creator.
והיה אם שכח תשכח את ה' אלקיך והלכת אחרי אלהים אחרים (ח-יט)
We have a general rule that the word "ויהי" denotes pain and anguish, whereas the word "והיה" infers happiness and joy. If so, then how can we possibly reconcile the words of the posuk: "והיה אם שכוח תשכח את ה' אלקיך" - “And it will be that if you forget Hashem, your G-d, and go after foreign gods and worship them .... you will surely perish.” Where is the simcha in that connotation?
The holy Rizhiner, R’ Yisroel Friedman ZT”L, interprets the posuk a bit differently. The only way to serve Hashem is with joy and true spiritual simcha. However, if one were to “forget” to serve Hashem with joy, then he will surely turn away from Him and eventually pursue other interests. Thus, "והיה אם שכוח תשכח" - If a person will forget the "והיה" - the joy and happiness - that is part and parcel of the Avodah of "את ה' אלקיך", he will tread down the dark path of Avodah Zara and he will destroy himself.
R’ Boruch’l of Mezhibuz ZT”L cites another explanation based on the words of the gemara (מגילה יב.), that the reason why Hashem sought to destroy Klal Yisroel at the time of Mordechai and Esther was because the people took part and thoroughly enjoyed themselves at the feast of Achashverosh. It wasn’t just that they were forced to be there; they were happy to partake and mingle with the heathens. Thus, it was a result of their joy in sinning that the decree was issued against them.
This, then, is the meaning of the posuk. It’s one thing if a Jew is forced to sin as a result of the terrible oppression that non-Jews force upon him. But "והיה אם שכח תשכח" - if he will forget Hashem - "והיה" - happily and willingly, then he will have crossed the line and gone too far. At that point, Hashem will destroy him and he will perish.