Flirting or Friendship? 

If you have a friend of the opposite sex that you are close too, here are some ways to know it has progressed into a budding affair. 

The friendship starts to influence each individual's mood. When they receive a digital, visual, written, or verbal exchange, they will have a mood-altering reaction. That reaction creates a rise in interest, distracts from pressing needs, lifts the spirit, and brightens their day. In other words, they feel better, lighter, and happier. Pressures disappear, discouragement vanishes, stress is gone. 

This is the "bright spot" in the midst of the mundane, the "sweet spot" where everything comes together. The individuals involved will actually start to think about this relationship when they need a boost, feel overwhelmed, or are just plain down. It becomes a highly addictive medication of choice that is easily available and doesn't create guilt (yet!).

This paragraph is from Dave Carder's book, "Anatomy Of An Affair" one of the best books I've read on how and why affairs, attractions, addictions happen, and how they make the offended spouse feel; and how long it takes to get over being betrayed, and what behaviors to expect.