Why affairs happen
Marriage often becomes routine. As the couple raises children, they may feel exhausted, and be less willing to give to each other, support each other and listen to each other with deep caring.
So when somebody, an outsider, comes along who shows interest in them, who makes them feel alive, the married man or woman may be poised on the edge of a male-female dance of excitement -- and danger.
For it is the newness that attracts the players to the affair, the fresh feeling of romance -- someone has come along who will support and understand them and give them the excitement they've been missing.