The problem is being so focused on the fantasy that it prevents a real relationship from happening. One reason is our fear that we are undesirable. It's better to believe that there's no one out there than to feel the pain of our insecurity. Perhaps it's a genuine fear based on abuse or other mistreatment in the past. We want to believe that love should be easy and when we run into difficulties we look elsewhere rather than solve the problem.
Intimacy is a challenge.
The responsibility and containment of relationship can seem unbearable at times. So we look for an escape by looking around and believe that it would be easier with someone else. We can't bear to think that love is somehow flawed. Unresolved anger or hurt, issues that may have begun with a parent, can stop us from jumping in with both feet. In this case we want to exert control over the relationship and not leave ourselves vulnerable to hurt again.