Dinner parties are often a trial for a single person, and this feeling is doubled when friends and family are trying to match them up with someone. Blind dates are awkward, and single people generally avoid them at all times. When a person cancels for the party, inviting another single person is often a ruse to introduce the pair. Being seated next to each other, the two have the choice of chatting or enduring hours of boredom while eating.
The majority of dinner parties are festive occasions where the group is celebrating an event or achievement. Being rude to other guests is unacceptable, and few people would fail to chat with the person seated next to them. This gives the host and hostess an opportunity to ensure the couple will have a chance to get to know each other, and they will also have an even number of guests to make the table complete.