If you walk in very drunk asking to be euthanised, one would hope that they would have to wait until you are no longer drunk to confirm you really want to be euthanised. Then, who decides on the waiting period?
Likewise if a traumatic event clouds your judgement (as some events can do), it's not unreasonable to make you wait until your judgement is sound. Then someone has to decide on the waiting period.
Potentially this can be studied statistically. Just collect data regularly after an event (such as binge drinking event) to evaluate how quickly people's judgement stabilises. That way we may find that the death of a spouse doesn't cloud the judgement, in which case the waiting period is 0.