A young rabbi once asked the Previous Rebbe for guidance in how to administer rebuke to his congregation. The Rebbe illustrated his answer by describing a sauna. In a sauna, it is customary to slap someone on the back with a towel. An incredible pleasure is derived from these "blows." The harder the smacking, the more pleasure is experienced! However, this is done only when that person's body heat reaches a certain temperature and he sweats profusely.
Just imagine if, the next day, the "slapper" comes along and smacks the other fellow on the back while he's walking down the street. This, of course, would be a most inappropriate time for such treatment, and the person being slapped would be greatly offended.
What is the difference in his reactions? In the sauna, the man was "warmed up" - ready and anticipating a good smack on the back. Out in the cold air, removed from the proper atmosphere and expectation, he is startled and upset. It's the wrong time and place.
In short, to have an effect on another Jew, you must first "warm him up" and make him receptive. Then you can "smack" him (i.e., provide him with guidance through constructive criticism). If you want to help someone with a problem, your intervention has to be at the right time and in the proper setting.
Related by Rabbi Sholom Ber Gordon, 1949