Attraction is magnified by an emotional connection. When one partner starts sharing himself or herself with another person, it chips away at the foundation of their relationship--and starts building a foundation for a new relationship.
Part of what makes a couple's relationship special is the information they share only with each other. Some of it is seemingly meaningless daily details, like how bad the morning traffic was or what they had for lunch. Other times it's deeper desires, fears and goals. As an emotional affair progresses, less and less of a person's sharing goes to his or her partner, and more goes to the affair partner.
In fact, not having sex may give the relationship even more power. You're able to idealize the other person and fantasize about what sex would be like. This only adds fuel to the fire. Just like primary relationships, affairs that start out slowly and build a connection before progressing to sex are often the most difficult to break off--and the most damaging to the other relationship.
Each person defines flirting differently. For some people, flirting is part of their personality and they do it, often unconsciously, with everyone from store clerks to family members. For other people, flirting is a sign of sexual interest and opens the door to temptation.

Flirting is generally harmless if the person doing it doesn't attach any significance to the behavior. Flirting alone isn't likely to interfere with a relationship. However, if flirting is happening because of sexual attraction and joins with those other ingredients of emotional infidelity--secrecy and increasing intimacy--it's cause for concern.
Some helpful questions to explore whether or not flirting is harmless include:
