I read a story a while ago. Sam is really unhappy and i can't really remeber the story other than sam feels really left out in the family. That is one scene I really remeber were Dean and Sam are talking and Sam says something about John doesn't love him. Sam says on his 18th he got nothing and on Deans he got the impala. Dean later speaks to his Sam about his and John says he did buy a car to fix but for Sam but sold it in instead. I loved the story I'm pretty sure it was on fanfiction.net

Also any other stories were Sam feels unloved and runs away or gets really hurt. Anything with lots of guilty and protective John and Dean.