Why does it say "Nimtze'u Devarecha va'Ochlem"?
Rashi: Initially, Your words were delicious to Me [and gladdened me], for I thought that they will heed me.
Radak, Malbim: When I found Your words in Nevu'ah, I accepted them willingly, like one who eats something tasty to him.
Malbim: Nevu'ah rests on a Navi via his preparation, When he wanted to prepare himself for Nevu'ah, he took a musician to rejoice, and afterwards Ru'ach rested on him, like it says "v'Lifneihem Nevel v'Sof v'Chalil" (Shmuel I, 10:5), "v'Atah Kach Li Menagen" (Melachim II, 3:15). However, Yirmeyah prophesized about punishments. He was always sad. He could not prepare himself for Nevu'ah. Nevu'ah came only when Hashem needed to send him. Then Hashem's word came to him; "Oz v'Chedvah bi'Mkomo" (Divrei ha'Yamim I, 16:27). Through the Dibur came Sason and Simchah, like one who drinks old wine, and he was prepared to receive Devar Hashem. After the Dibur, he returned to be stunned and sad about the Dibur itself, which was full of visions of Churban and punishments. Nimtze'u implies that the Dibur came like one who found a Metzi'ah without thinking about it.
Why is it written Devarecha, and we pronounce it Devarcha?
Radak: It is written [plural] based on the individual [words], and it is pronounced [singular], like the Klal.
Why does it say "va'Yhi Devarecha Li l'Sason ul'Simchah"?
Malbim: The Dibur aroused Simchah in me, which is necessary for Nevu'ah.
Why did he say "Ki Nikra Shimcha Alai"?
Radak: [I was glad that people will say about me 'he is Navi Hashem', and other Nevi'im will be called Nevi'ei ha'Ba'al.
Malbim: It was aroused from above. The Simchah did not come to me before the Nevu'ah.
