In one of her studies of Vayechi, “Jacob’s Testament,” Nechama Leibowitz* discusses Joseph’s reluctance to swear to Jacob’s burial wish:
The Midrash aptly explains the difference between Joseph’s behavior and that of Abraham’s servant [when asked to swear, regarding finding a wife for Isaac]:
Said Rabbi Isaac: The servant acted servilely and the freeman as a free agent. The servant acted servilely, as it is said: “And the servant put his hand…” Whilst the freeman acted as a free agent: “And he said, I will do as thou hast said.” (Bereshit Rabbah* 96)
A servant has to do the behest of others….A free agent however is only bound by his conscience, and chooses his own actions in accordance with his own freely arrived-at decisions.
Malbim** makes a similar distinction…It was better for him to do it out of his own free will, rather than be bound by oath. In the latter instance, he could not take the credit for fulfilling his obligations freely.
This explanation may help us understand Biblical and Rabbinic disapproval of vows. Man should rather conduct himself as a free agent rather than be bound by external bonds…
The topic of vows is a complex one in Judaism and offers an interesting path to follow. Here are two basic articles on vows, vowing and oaths: one from R. Louis Jacobs at My Jewish Learning and one from the Encyclopedia of Judaism.