Pretty soon, you find yourself glowing every time you spend time with this person. Any contact with the person becomes as potent as a drug addiction.
All those tingly feelings and the fantasies that perhaps a "perfect love" can really exist isn't destiny knocking -- they're caused by "love chemicals" in your brain.
You become friends with the sexy co-worker and decide to carpool to work together. You're married, or engaged, or you're in a committed relationship. Emotional sex is a friendship that escalates into something that feels the same as romantic love and can manifest itself in numerous ways -- physically, romantically, emotionally, lustfully, verbally, or virtually.
You become "friends" with an ex on Facebook and reminisce about the past. You spend hours thinking about them and your heart races whenever you see a text from them. You tell yourself it's ok because you're not really cheating, you're just chatting. Friendship becomes emotional sex when the feel-good brain chemicals and hormones that are released when even thinking about that person take over.