Sherlock claims he doesn't drink because it slows him down. He's lying – it speeds him up.
His first month at university he went too fast, coasted too far with a classmate in a haze of gin. Sherlock only craved the approving stroke of his hand, warm weight against his chest. But he faked the rest and woke up full of contempt, poisoned by the power his lies gave him over his partner. He hadn't wanted that.
He does want the safe pressure of John's arms, but it would destroy him to wake up hating his only friend. Sobriety's much safer.