Takes place before end of Season 6 but after Sam is re-souled.
Until age thirteen Sam Winchester had been known simply as Sammy Winchester. As a teenager he thought the name was just a reminder that he was the baby of the family, but it wasn't.
During his time at Stanford no one called him Sammy, though Jess did a couple of times. It wasn't like he'd told people not to address him by that name, but no one seemed to call him anything other than Sam or Winchester.
When he got back on the road with his brother 'Sammy' was used to annoy the hell out of him, or so he thought. When his girlfriend went up in flames along with the rest of his dreams 'Sammy' came out of Dean's mouth to comfort him, though Sam didn't really realize it at the time through his grief.
Through the years Sam warmed back up to the nickname. He yearned to be called by the name he'd used when he and Dean weren't vessels, John wasn't dead, he was ignorant to the demon blood coursing through him, and he'd never been tossed into Hell with Lucifer, Michael, and his half-brother Adam as company. While his soul was missing Sam absently noticed Dean's lack of the word 'Sammy'. At the time he'd thought nothing of it; he had more important things to deal with.
But he was out now-and Dean was calling him Sammy all the time.
Sorry this has been so lately updated. And sorry again to all of you expecting a Shades of Gray/Legacy of Hunters update; it ain't happening for a while. Unfortunately I've lost all my muse while working on a different project.