Your partner can feel like the enemy
The person with whom you’re having the emotional affair has no control over you. He can’t possibly attempt to tell you how to behave or really have any expectations of you. By contrast, this starts to make your partner (who does have some input in your life) feel like an enemy or authority figure to you. The person you’re emotionally cheating with provides this la-la land escapism world where you have no responsibility to anyone, but still derive some pleasure.