In every important area of life we do better when we have a coach. A coach has a more objective view and will be able to point out ways that we can improve. Even if someone else doesn't have more knowledge, since they are more objective, their input could be valuable.
A serenity coach could be someone who is a model of serenity. Or it could be anyone who observes us and points out to us when we need to return to a more serene state. Others view our facial reactions, which we don't see unless we are looking into a mirror, and therefore observe the biofeedback that is expressed on our faces. It makes sense that if you have an opportunity to benefit from someone who agrees to be your coach, that you shouldn't pass up the opportunity.