When partners feel some 'relationship threat' it is difficult to feel secure.
Relationship threats that plague many are the fear that:
"I will let you down or disappoint you."
"I will evoke your anger."
"I am unworthy of your love."
"I do not deserve you."
"You won't be there for me when I need you."
"You will eventually get fed up with me and leave."
These are attachment threats. Attachment threats may be experienced because of unreliable or hurtful behavior in the relationship. However, it could also exist because of anxiety experienced in earlier relationships for one or both individuals that is reinforced by behavior in the current relationship.
When fear revolves around attachment threats there are two basic behavioral tendencies we typically see. Some people make their distress more visible by "turning it up." People who turn down their distress typically do so by avoiding emotions or hated interactions and withdrawing from an interacting when feeling threatened. People who turn up their visibility tend to move toward their relationship partners when threatened by irritating contact and attempting to get through to them any way they can.
For couples, anxiety is a troubled form of distress that can have problematic implications for the relationship. Fortunately, if the specific threat or fear that is fueling the anxiety can be identified, anxiety can settle down
When people are able to express fears directly and experience their partner's sincere reassurance, the threat and attachment distress can diminish.
Taken from Emotionally Focused Workbook
Katios-Lilly & Fitzgerald