When a man seeks to anul a vow, the court must find some
cause for regret of the vow. The court must not hold as a
reason that the vow is derogatory to his mother's dignity, but must
find some other source. Why? Because this is too easy, and allowing
it would encourage people to make vows lightly because they can be
easily anulled. (Making vows was generally discouraged.) (13b)
(I would have thought that because of kibud av v'eim, honor
your father and mother, there would be strong reason to accept
"this dishonors my mother" as justification. I suspect the rabbis,
in addition to trying to make it harder to make vows, are also
trying to make people less casual and more thoughtful in evaluating
parental honor.)