Operating on the personal malaise an addict feels, drugs give him an artificial sense of self-sufficiency that removes what small motivation he has for complicated or difficult pursuits. Similarly, Vicky and Bruce came to each other originally in a state of ennui, without interests that extended deep enough to engage them. Their dependency, like the drug addict's, could only increase as other concerns dropped off. Vicky and Bruce organized their environment so totally around each other that there never was the slightest threat of a breakup. The couple had at some cost resisted even temporary separations, as when Bruce resigned himself to a school he didn't like in order to stay with his fiancee.
Lovers often do suffer withdrawal symptoms when breakups occur.
With Vicky and Bruce, the passion for staying together was in large part a fear and avoidance of withdrawal. Addiction to a person is just an extension of the social side of all addictions. People who share an addiction form a private world for themselves. For example, a group of people who are addicted to the same drug tend to give their experience a collective interpretation which is incomprehensible to those outside the group. They are not concerned with this discrepancy because only the approval of other group members matters to them.