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Sam has known plenty of people with bad habits. His Dad and Bobby always drank too much, Pastor Jim recited scripture whenever he got nervous and Dean blasted cock-rock through the Impala's shitty speakers at all hours of the day and night. Demon blood addiction is worse than all of those things combined. After God gave him a free pass and a clean bill of health, Sam honestly thought he'd never stoop that low again. Swore that he wouldn't. Sure, he'd found it hard to stay on the wagon at first, but there were very serious, world-ending reasons why it was a shitty idea to give in to his baser urges.
Even though he'd drunk enough blood to host Lucifer, the soulless version of him that was wandering around topside, while Sam was stuck in the cage, must have figured the addiction was a liability that he didn't need. Because Sam wasn't jonesing for a hit once he got his flesh back. Whatever his reasons for getting clean, Sam was grateful for it, because he'd not thought about, or touched, the red stuff, for almost five years.
But then Dean became a demon, and all Sam's achievements on that front were shot to Hell. He didn't even notice what Dean was up to at first; a testament to how naive he'd been. Dean had been purposefully injuring himself in Sam's presence, drawing blood to tempt him. Careless with kitchen knives or when cleaning weapons - letting a monster get the drop on him while they were on hunts, just enough to get thick red blood flowing down his shirt or jeans, splattering across the floorboards at his feet.
When that didn't work, Dean got himself banged up enough for Sam to fuss over him, because demon or no, he was still Sam's big brother - and then he made his move, bleeding into Sam's mouth and clamping down on his nose, forcing him to swallow. It was the same shit those pissed off hunters tried when they found out he'd started the apocalypse - but Sam didn't have as much luck resisting this time. Dean had important advantages they didn't; the strength and abilities of a Knight of Hell, and Sam's unconditional love.
Somewhere along the line, he'd grown to accept this life, this fucked up relationship with his brother, which was still the deepest, most destructive and most important relationship he'd ever had. He'd admitted to himself what had always been the truth he was to stubborn to see - this was the best a he was going to get out of life. Time and again he'd had the chance to take another path, another life, even another person, to rest all his hopes and dreams with. But in the end he'd always chosen Dean, and he knew he always would. No matter how much he tried to tell himself he'd loved Jess, and Amelia, they were just words. Dean had asked him to leave them behind, and each time, Sam had dropped everything and gone running after the person he loved most in this awful universe.
So maybe he could be excused for taking too long to acknowledge the bloodlust that had taken over his brother, but now it was far too late. Sam was little more than a dependent junkie again, dying for his next fix, begging his brother for nourishment like an animal whining for scraps. Not at this current moment in time though; today he'd been good, followed Dean's orders, been a good soldier, so he was getting his 'treat'. His reward for good behaviour. Dean was feeding him straight from the tap, letting Sam drink from his arm, enough to get him woozy from the rush, senses spinning from the heady sensation of power, greed and muted rage.
Sam knew he was moaning, slurping while he sucked, trickles of blood running down his chin as he suckled like an infant on the teat. He just didn't care. Blood was his life now, blood was everything, and Sam wanted it all, needed it to stay sane. He'd tear himself apart to get just one drop more. But right now there was no need; Dean was letting him have his fill, brushing back what he called Sam's 'curtain of hair', so that his face was exposed to the dim light. Sam squirmed in his brother's hold, wriggling as he tried to get even closer, even though they were as entwined as two people could possibly be. It wasn't enough; Sam wanted to crawl inside his brother's skin and live there forever, feasting on the cocoon of his blood. Sam groaned at the thought of it, hot red blood sticking like tar in his teeth.
Dean hushed him when Sam started to whimper, clinging to his brother's arm, digging blunt nails into Dean's skin, afraid his precious food source was going to be whisked away from him. It should have been creepy - should have disturbed him when Dean cooed at him, stroking gentle fingers through his hair, but it only served to make Sam feel safe, content. He knew his big brother would take good care of him - he always did.