Top three reasons that women cite for having an affair are usually number one, feeling distance or disconnected from your primary partner. Number two, developing a strong emotional bond with an affair partner and number three, wanting affirmation of their sexual desirability. So it's very important for a woman in a long term relationship particularly that her partner gives her continuous affirmation of her attractiveness and that would consist of compliments, flowers, dates that is continued to make her feel attractive, desirable and romantically interested in her.
For example, a woman may feel very distant and unable to communicate with her husband yet develop a very strong bond with a co-worker with whom she can share all sorts of intimacies that her husband either wouldn't understand or wouldn't have time for. Over time she may feel stronger and stronger about this relationship and desire more than just friendship, desire to test drive that relationship as an alternative to her existing relationship.
When a woman feels that her partner has lost sexual interest in her or no longer finds her as attractive as he used to. No longer pays attention, no longer hugs her, doesn't french kiss her anymore, many times she will feel like exploring those things outside the relationship to assure herself that she's still got it, that she still has that romantic and sexual desirability for other men.