Dr. John Huber & Mainstream Mental Health Radio present - http://mainstreammentalhealth.org/  

As a physician, couples therapist and relationship expert, Dion Metzger, MD.                                                                                                             

1. How does the brain change when someone is in a relationship? Intense attraction and feelings of connection between two people can actually cause biological changes in the brain. When a person is in a relationship, their brain release specific chemicals including dopamine and oxytocin. This can lead to a sense of euphoria and a strong sense of attachment to their partner. They can act differently by wanting to spend more time with their significant other, participating in romantic gestures and being more expressive about their emotions. 

2. Can being in love, or being in a relationship, change you -- or even affect your brain chemistry? Being in love can change you by making you more vulnerable and conscious of your emotions. I have also seen it heighten sensitivity in people who may have been emotionally withdrawn before. There are not exactly permanent brain chemistry changes but it can definitely lead to more careful behaviors for those who have been in love before. They want the euphoria feeling again but are guarded about letting their guard down, especially if the last relationship ended with heartbreak. Its a form of classic conditioning. You're more hesitant to talk the stove if you got burned before.