"...Because you did not serve Hashem your God - in happiness and goodheartness, and an abundance of all.
You will serve your enemies, whom Hashem will send among you - in hunger and thirst and nakedness and lack of all." (28:47-48)
Chassidim love to quote the first verse, to show why you must always be happy and smiley and touchy-feely when doing mitzvot. After all they say, it talks about people who kept all the mitzvot, but in a dour Lithuanian manner, and God exiled them for it!
Fortunately, this is a total misinterpretation of the verse. Look at the next verse, and the parallels between the two verses. Serving God vs. serving enemies; abundance of all vs. lack of all. In each verse, the object of service comes first, followed by conditions in which you serve them.
In the second verse, "Hunger and thirst and nakedness and lack of all" does not describe your attitude - how can you have a hungry and naked attitude? - but rather the physical conditions you are in. Similarly, "happiness and goodheartness, with an abundance of all" in the first verse must describe the conditions you were in. You were rich and successful and living at ease, yet you did not bother to serve God. Thus you will now be punished, and lose your comfort and wealth.
Thus, the first verse is best translated as "...because you did not serve Hashem your God - WHEN YOU HAD happiness and goodheartness, and an abundance of all."
Happiness is of course legitimate. There is a verse in Tehillim which says "ivdu et Hashem besimcha" (100:2). But there is also a verse which says "ivdu et Hashem beyirah" (2:11). The two have to be balanced. Each emotion is appropriate in some situations - and inappropriate in others.
"These are the words of the covenant which Hashem commanded Moshe to make with the children of Israel, in the land of Moav - besides the covenant He made with them in Horev [=Sinai]." (28:69)
"These words" means chapter 28 - an actual specific text which was passed back and forth, or signed, or whatever else was appropriate, in order to make it into a binding covenant. What is the comparable covenant at Sinai? The whole giving of the Torah was a sort of covenant. But more specifically, by comparison with Ki Tavo, it would seem that that the covenant at Sinai was a particular written text.
Luckily, we possess a very similar text which we know comes from Sinai - parshat Bechukotai. There we have a blessing and curse which are extremely similar to the ones in Ki Tavo. It seems that they too form a written covenant. They date to Sinai, even though due to the verbosity of Sefer Vayikra they come long after the main Sinai story.
God and the Jewish people talked to each other, promised, warned, agreed, and discussed many things at many times. It seems that all of this is condensed and crystallized into a single written "contract" at the end of each story. The rewards, punishments, and promises - in short, the terms of the contract - are recorded in the form of a covenant. There is one such covenant at the end of Vayikra, at the end of the Sinai story. There is another in Ki Tavo, when a new generation finished its desert experience and prepared to enter Israel.