While the parties might expect the mediation to be a one shot deal, there are a lot of things that can prevent a deal from happening in the first session: somebody may not be well-prepared, somebody who should be involved may fail to attend, one or both sides may decide they need more information before they can make a settlement offer, a party may receive information during the course of the mediation that they need time to process. When any of those things happen, and we cannot work around it, I try to get the parties to agree to return for another session in a week or a month. But settlement momentum is sometimes lost during that break, and the parties sometimes get bogged down in wasteful litigation activity. Offers sometimes come off the table, and parties may move farther apart. If instead parties had the expectation that the first time they meet their mediator--just like the first time they appear before their judge, or the first time parties in counseling meet their therapist--might not be their last, they might not view the initial session as the make-or-break activity that is often the case now, and they might be less likely to take actions harmful to the settlement process during the time between sessions..
A cynic might point out that of course it is in the interest of mediators to encourage parties to come back for multiple sessions. And that might be true, just as it is true for psychiatrists or ski instructors. But I'm also quite happy to see a case resolve in a day or less. In a lot of cases, I know that the first session is all I'm going to get, and I will make every effort to get the case resolved in the time we have. What I don't like to see, however, is for people to give up on mediation just because they did not succeed in settling their dispute in the first meeting. In some cases, there are obstacles that can make it too difficult to do that, and the parties just need to come back and give it another try. The settlement track should stay open, and parties should be willing to continue as long as negotiations are productive. A persistent attitude will prevent the loss of settlement momentum.