Not Being Too Machmir

Rabbi Moshe Dovid Valley, in his commentary on Chumash, has an interesting explanation for the releasing of vows. The Torah empowers a person to make an oath or a vow which will prohibit him from engaging in certain activities. Once he makes his personal commitment, the person is obligated to live by it. Not doing so is a violation of a Torah commandment.
However, even though the Torah does not say so explicitly, there is a tradition that one who makes a vow can go to a rabbi or Beth Din and have his vow annulled. Why would it be permitted to annul a vow?
He explains that Hashem understands how difficult it is for a person to abide by his commitment to be strict upon himself according to the terms of his oath. Rather than have the person transgress his oath and suffer the consequences for doing so, it is better to institute a way for him to be freed of his vow.
It would seem that the spirit of this Mitzvah would explain why Chazal say that כח דהיתירא עדיף. When a rabbi is presented with a Halachic dilemma, it is preferable for him to be able to find a reason to permit something than to prohibit it. The Torah does not want to be creating new issues for people to transgress and then suffer consequences.