Rashi and Seforno: After what Shechem did, one would expect Yaakov and his sons to be angry. So Shechem and Chamor found it necessary to reassure the townspeople that this was not the case, or that they were willing to overlook his sin, since it was to their advantage to join forces and to merge into one nation. ¹

Ramban: The townsfolk, who noticed how distraught Yaakov's sons were, and the angry looks on their faces, assumed (correctly) that they hated them and sought revenge. Perhaps they even took steps to safeguard themselves by adding doors and bolts throughout the town - since Yaakov and his sons were mighty warriors. That is why Shechem and Chamor needed to convince them that they were wrong.

Oznayim la'Torah: In keeping with "Va'Yavo Yaakov Shalem" (33:18), ² Shechem was informing the people that Yaakov and his sons were in perfect health, wise and astute and wealthy - and therefore perfectly acceptable to merge into one nation with them.

Ha'amek Davar, citing Moharitz: Through them, we will be complete! Every country lacks things that grow elsewhere. We need people capable of buying the excess grain of our land and bringing what grows in Chutz la'Aretz. The Ivrim are more capable of this than we are. ³ The land is wide, and grows extra. They will market it!

Rashi: By pointing out that, due to the business opportunities that such a merger would open up, it was to their (the people of Shechem's) advantage, too.

Refer to 34:9:1:2.

Malbim: They pointed out the gains they will get if Yaakov's family stays. (a) They are Shelemim with us, and will help us against any enemy. (b) They will dwell with us - until now, the land was desolate, for there were few residents. They want pasture for their flocks! (c) They will do business with us. (d) They want to intermarry with us.