More importantly, other commentors here have already admitted to flagging this entry. The way flagging exists now rewards one-sideism and partisan behaviour - all it takes is a relatively small group of discontented people to take down a story that is otherwise interesting to the vast majority of posters. A counter-flag option would balance things.
That's not accurate, because if a story is interesting to the vast majority of users, it will get lots of upvotes—and lots of upvotes is enough to defeat a small number of flags. In that sense, we already have the counter-flag option you're arguing for.
Stories don't always get the chance to gather the sufficient amount of up votes before being nipped in the bud by dissatisfied flaggers though, depending on the time of day. Some of them, like >>46357887 , clearly had great interest here and got a large number of upvotes that was, nonetheless, insufficient to prevent the flagging.