This fear leads them to build mental fortresses of ideas with which to defend the realities their egos are creating from their own personal desires. Ideas their egos don't believe in are held outside those fortresses. Many of the beliefs they are given by their cultures will run counter to the fulfillment of their personal desires. Their suppressed desire will lurk in the mental background of their psyches looking for a mechanism to release itself from the ideas containing it.
Intoxication offers the promise of such a release.
Drug and alcohol intoxication can loosen those mental idea fortresses enough to provide a temporary escape for the energy arising from the individual's personal desires. As it does this, alter egos are created depending on intoxication for their survival.