[Broadly speaking]
What's going on is that a part of the physical brain, having recently had a 'hit', and having correlated the hit with a behaviour, sends out the message 'do [insert behaviour] again'.
That's all. It's a signal.
The signal, however, comes 'wrapped' in flavoursome packaging: the recollection of pleasure and the anticipation of pleasure, with the associated anticipated 'pain' of such pleasure being withheld.
Here's the strange thing. You can be absolutely fine, then a withdrawal signal is received, and suddenly you're in 'pain'. Pain is not the description of the state you were in before the signal. It's the description of what is induced by the desire accompanied with the prospect of not giving in to it.
You can be perfectly comfortable, and then, suddenly, you think of using, and now you're really uncomfortable. The discomfort is not the description of the non-using state. It is the description of the state of someone who is not using yet who has thought of using and is conflicted about it.
In other words restlessness, irritability, and discontentment are not the description of sobriety, the state of being without alcohol.
They are the description of the state occurring between desire and fulfilment of the desire.
If there's no gap because the fulfilment of the desire occurs straight away, there's no time for restlessness, irritability, and discontent to develop. As Grady O'H says, you have to withstand the desire for the first drink to realise you have the obsession [in the broad sense of the pre-drinking frame of mind rather than the specific sense of the associated delusion], and you have to withstand the desire for the second or subsequent drink to realise you have the craving. If you immediately give into the thought of a drink then immediately give into the desire for the second and subsequent drinks, you never experience the obsession or the craving as a lived state.
This explains why Silkworth, in the Doctor's Opinion, refers to alcoholics being restless, irritable, and discontent (he's describing alcoholics who are trying to avoid the first drink) yet no such state is described in respect of the certain American businessman (perfect mental health), the man of thirty (happy as Larry), the jaywalker (he just jaywalks whenever he damn well pleases), Jim (ordinary day), Fred (perfect day). None of them held out, so none experienced the pain of doing so.