The night Ford Prefect started drinking was the night he realized that he wasn't going back home.
True, he didn't really have a "home" anymore. "Home" was something he had come to understand as whatever place he could return after a day of exploring and causing trouble on the planet he was currently visiting. But, paradoxically, this lack of home had thus given him the sense that everywhere was home to him as long as he knew where his towel was and could get off the planet whenever he wanted to seek new adventures. Now he was stuck, and on the Earth, of all places. The mind-boggling dullness of this planet, coupled with the fact that the exciting chaos he had come to love and thrive in was gone (seemingly forever), was what drove him to drink.
He'd been previously familiar with alcohol before now, of course. One can't be a galactic hitchhiker without developing at least a passing familiarity with all the best-known forms of intoxicants that the galaxy has to offer. (Well, it's possible that one can, but this leads to one being looked down upon by other hitchhikers as "not fun".) But he'd never consciously made it a part of his life. He'd heard of other people "drowning their sorrows", and this, being the hugest loss he'd yet sustained in his life, definitely qualified as a sorrow. What could he do with it but drown it?
There was a bottle of red wine he kept for when visitors came to his flat. He'd learned that humans have such a concept as "hospitality", and offering people drinks seemed to fall under that category. With a regretful sigh, he opened the cupboard in which he kept the wine and poured himself a glass. He didn't drink it right away. He spent a good half-hour staring at it, really considering what he was about to do. It was difficult. Ford Prefect wasn't the sort of man who carefully considered the consequences of his actions, but even he knew that he wasn't just about to consume some alcohol. He was going to consume some alcohol as a deliberate reaction to a situation and thus impose his own solution onto it.
But what was he solving, really? What did this actually solve? Nothing, and he knew it. He would spend the rest of the evening intoxicated, and he'd probably wake up very sick in the morning, and that's what he would accomplish. Hardly an accomplishment of any kind. No, it really wouldn't be an accomplishment. He knew it. It would be problem that he'd deliberately pile onto another problem. It would be the exact opposite of a solution. It wasn't what he wanted. It was a painfully bad decision, and he knew it.
As Ford downed the glass of wine, not even bothering to note its taste or anything about it other than where it would soon get him, he wondered if this was how most people came to be alcoholics. As he downed a second glass, he didn't care about this question. As he downed a third and a fourth and a fifth, he didn't care about anything.