Parashat Chayei-Sara describes the mission undertaken by Avraham's servant to go to Charan and find a suitable wife for Yitzchak. As we know, the servant declares that the girl whom he would ask for water and would give for both him and his camels will be the girl destined to marry Yitzchak. The servant tells all this to the family of Rivka, who indeed offered water to the servant and his camels. The family, expressing their consent, responds: "The matter was decreed by the Lord; we cannot speak to you bad or good" (24:50). What do they mean when they say, "we cannot speak to you bad or good"?
Rashi explains "bad" and "good" as referring to the validity of an argument they could have potentially raised against the match. Not even a "good," or valid, argument could get in the way of God's will. Since Rivka's marriage to Yitzchak was clearly willed by the Almighty, no reason, not even a seemingly valid reason, can interfere.
Rabbenu Yossef Bekhor Shor and the Seforno explain differently, claiming that "good" refers to the family's consent to the marriage, whereas "bad" means their refusal. The family tells the servant that given the divine prof the match, they can say nothing; even expressing their consent would be superfluous, insofar as Providence has willed it. This approach appears to have origins in the Pesikta Zutreta on this verse, which reads, "One can understand 'bad'; but why [could they not speak] 'good'? Rather, the good is not from us, but was rather decreed from God." Meaning, any expression of consent on their part would imply their independent control over the matter; since God decreed this union, they cannot say anything, even to express their consent.
A novel, and perhaps homiletic, interpretation of the verse is suggested by the Chatam Sofer. "Good" and "bad" here refer to Rivka's qualities. Her family tells the servant that given that God clearly wishes for this marriage to occur, they do want to say anything that may discourage Avraham's servant from going through with it. Therefore, they will not say anything negative about Rivka, but nor will they say anything positive about her. In creating a marriage, a balance of qualities and characteristics is necessary. If the husband is particularly generous and gives out money freely, then generosity on the wife's part - an otherwise laudable quality - becomes a negative trait. Two free-giving donors in a marriage will leave the cupboard empty at home. Therefore, Rivka's family refrained from saying anything about Rivka - even her good qualities, lest this discourage the servant from accepting the match, which would then undermine God's plan.