Please see Dr. Sternberg's "Triangle Theory of Love".  Three essential ingredients are essential for Total Love.  Intimacy, Passion, and Commitment.  Usually, affairs are about Passion and perhaps some Intimacy.  Usually those engaged in affairs (relationships while married) are not looking for commitment, they have all the commitment they can stand at home.  They typically are looking for Passion only, especially if their best friends provide for their Intimacy. Behavior is a powerful indicator of these truths.