Rashi: In praise of Yosef's brothers, it implies that they were not two-faced, pretending to be his friends when in fact, they hated him.

Seforno: Although they had to communicate with him in his capacity as manager of their father's estate (refer to 37:3:4:2), they could not bring themselves to talk to him in a brotherly manner.

Kol Eliyahu: One blesses someone who leaves with "Go l'Shalom," and not "b'Shalom" (which is a bad sign - Berachos 64a). Because they hated him, they could not say the word 'l'Shalom.'

Ohr ha'Chayim: Normally, they would have argued with him and rebuked him, and their envy would have departed. However, they saw that Yaakov loves him more than all of them together, and that he publicized this with the tunic. Now, they could not debate with him to make Shalom, since Yaakov would always vindicate him (Yosef could always 'prove' that he was right)!

Malbim: Normally, words of Shalom assuage hatred. However, hatred based on envy and thinking that he is a Rasha lying in ambush, when he [appears to] speak peaceably, this increases the hatred.

Ha'amek Davar: They could not bear his words of Shalom, for they thought that also he hates them, but hides his hatred. "Yachlu" is from Yachil, a measure; they did not have enough tolerance.

Ohr Yechezkel (Darchei ha'Avodah, p.36): It was not simple hatred of brothers. Everything is in Hashem's hands. ¹ We find that right after they sold Yosef, they regretted this and wanted to return him!

Oznayim la'Torah: Now that Yaakov actually displayed his favoritism of Yosef, the brothers were afraid that, like Avraham, who gave everything he had to Yitzchak and sent the sons of his concubines away from Yitzchak, ² he too, would give everything he had to Yosef and send them ³ away.

Ramban, Ha'amek Davar: The sons of Bilhah and Zilpah hated him for his Lashon ha'Ra about them. Bnei Leah hated him because Yaakov loved him more than them.

Malbim: a. They knew that of Avraham's sons, only Yitzchak was chosen, and of Yitzchak's sons, only Yaakov. They saw that Yaakov loves only Yosef, they feared that he sees Yosef as primary, and the rest of them are the shell. The robe supported this; it designates him for Avodas Hashem, like Bigdei Kehunah. b. Because he spoke Lashon ha'Ra to sully them in his father's eyes. c. Because his dreams were proof that he wanted also kingship!