Our oceanic need for love, warmth, not to be hungry or afraid or bored, to be affirmed, can transmute into the desire of a single signifier: for example. alcohol, drugs, sex, or gambling. 

When a baby cries, it may need 'warmth/milk/dry/mumma/sleep/not scared/not bored/...' - but will be offered a bottle. We see the baby grudgingly accept the offer and allow itself to be soothed by it, even though the bottle is only a partial solution to its need. 

For the baby, there is, apparently, no meeting its deeper, so-much wider, need. And when the baby enters language, it finds there is no word for it anyway, so it literally cannot ask for the thing it wants. 

It time, the child acquires the names of partial solutions, and these become its objects of desire. This is the universal human attempt to resolve the dilemma of the unspeakable.

Desire arises when language gives us the illusion of naming need.

The distinguishing aspect of a compulsive client, is that they have located one or more specific signifiers as the solution to this dilemma of the unspeakable.

But if you ask them what they want, we understand why they cannot say.