Why is emotional infidelity so painful?

Emotional infidelity is painful because it violates the emotional bond that two people have spent time cultivating in order to have a romantic relationship. Once someone has been violated in this way it can take them a very long time if ever to rebuild the strength of the trust that they once had in the other person. A relationship without intensity will eventually file itself in the failure category. This is because people cannot sustain a functional relationship without some degree of emotional intimacy being involved.