You, Sam, and Dean had known each other since you were kids. Your dads were both hunters, and that's how you three met. You and Dean got along really well despite the age difference. You were younger than both Dean and Sam.
You began hunting with Sam and Dean when their dad disappeared and have been hunting with them for four years since. There is now a whole mess of things. Dean just got back from Hell, and Sam is addicted to demon blood.
But that doesn't change the fact that Sam has been so desperately in love with you for the longest time. But, he's never said anything because he was convinced you were in love with Dean. Sam was insecure. Extremely insecure. His older brother was handsome, charming, and a ladies' man... but Sammy..? Sam was tall, awkward, and broad. At least that's what he'd told himself.
One day, Sam and you were in the Impala, driving, working on a case. His emotions and feelings had been festering so long he was reaching a boiling point, and the demon blood in his system wasn't helping the inner turmoil one bit...