I
"You shall rejoice with all the goodness that Hashem, your G-d, has given to you and your household" (Devarim 26:1). We do not recite the recitation over the bikurim except at a time of joy, from Shavuos until Sukkos (Rashi).
The last pasuk of the recitation includes the word hineh, which means with joy (Sifrei, 26:10). Without joy, the mitzvah is incomplete (Malbim).
"The joy that a person experiences by the performance of mitzvos and the love of Hashem Who commanded them is a great service. One who refrains from this joy is worthy of punishment, as it is said, 'Because you did not serve Hashem your G-d, with joy and with goodness of heart' (Devarim 28:47)" (Rambam, Lulav 8:15).
The horrible curses (28:46) are attributed to a failure to serve Hashem with joy. This is surprising, especially since a different, more understandable, reason is given earlier - "If you will not listen to the voice of Hashem, your G-d, to keep and perform all of His commandments and decrees" (28:15).
Apparently, the joy of mitzvos is a critical and indispensible ingredient in serving Hashem. If one performs mitzvos out of habit, he is not really listening to the voice of Hashem, but rather engaging in rote ritual.
Even service of the heart, which cannot be entirely thoughtless, is worthy of punishment if incomplete. Loving Hashem which does not lead to joy is, by definition, insufficient. "Their fear of Me is like rote learning of human commands" (Yeshaya 29:13). This, too, reflects a heart distant from Hashem and is punishable (14).
II
The pasuk (28:47) concludes, "merov kol", out of an abundance of everything, while you still had all that is good (Rashi). Even though you were blessed by Hashem with prosperity, which should lead to joy, as the parsha of bikurim notes, you failed to serve Hashem with joy. Alternatively, because you were blessed with prosperity, you failed to serve Hashem with joy. Wealth leads to haughtiness and forgetting Hashem (Devarim 8:13, 14).
Remarkably, Rambam (ibid.) contrasts service of Hashem with arrogant self-centeredness. Rejoicing humbly before Hashem, as David Hamelech did, is true greatness and honor. One who is blessed with an abundance of everything - wealth, power, etc. - and continues to serve Hashem with joy and humility, is the exemplar who is worthy of blessing and not curse.
Just as serving Hashem without joy can lead to punishment, so can excessive joy, even when cloaked as a mitzvah. "When one eats, drinks, and rejoices on a holiday, he should not be drawn into excess wine, levity, and frivolity, and say that more of them is a greater mitzvah of simcha. Intoxication, excessive levity and frivolity are not simcha, but madness and folly. We are not commanded to engage in madness and folly, but rather in simcha that contains the service of the Creator, as it says, 'Because you did not serve Hashem with joy.' Service of Hashem is with joy. It is impossible to serve Him with levity, frivolity, or intoxication" (Rambam, Yom Tov 6:20).
Unfortunately, we are influenced by a society that engages in these negative activities. We must always, but especially during holidays, avoid them, and experience a true simcha shel mitzvah.
III
The phrase "merov kol" can be interpreted in yet another way. "Rov" means majority. "Merov kol" can mean a majority of everything. Some individuals who have been blessed with a majority of everything focus on the minority that, in their perception, is missing. This focus precludes serving Hashem with joy.
In reality, no person has "everything." Life is filled with challenges, which are viewed as problems or shortcomings. The wrong attitude can prevent one from serving Hashem with joy, and even lead to being angry, or upset, with one's fate.
One must accept that what he has, and what he lacks, come form Hashem Who knows what is best for every person. "You shall rejoice with all the goodness that Hashem, your G-d, has given to you" (Devarim 26:11). Hashem is your personal G-d Who has chosen what goodness to give to you, in particular.
This idea is immutable, and was true even when Am Yisrael lived in dire poverty and suffered persecution. How much more so today, when we live in wealth and freedom. Unfortunately, at a time when we have a significant majority of everything, some are focused on the perceived missing minority. This can border on a lack of faith in Divine Providence, and prevent serving Hashem with joy.
The curses are read before Rosh Hashanah, expressing a prayer that the year and its curses should end (Megilah 31b). If we serve Hashem with joy, we will merit a new year filled with Hashem's blessings.