We scouted through the internet, spoke to some experts and observed a few couples and realized there could be 3 reasons mainly for such a relationship to happen.
If the affair is about living out a ‘fantasy’, then remember that the ‘other’ self can only exist in the bubble of secrecy, where it is untouched by the pressures of the real world.
If you look around now, you will find those trying to save their marriage and those trying to run away from one- by satisfying their needs from an affair outside marriage. Is it the emotional void they are trying to fulfill? Is it unethical or being disloyal?