How do I disable the Require Win/Loss Reason field when closing my opportunity?

The Close Opportunity Win/Loss Reason Required profile option (MOO_CLOSE_WINLOSS_REQD) lets you specify whether salespeople must enter a win/loss reason when closing an opportunity. This profile option also applies at the revenue item level.

You can change the default value from Y to N so that salespeople won't have to enter a reason when an opportunity is won or lost.