Spoiler Alert thru season 5
Disclaimer: I do not own Supernatural; if I did my stories would be on the show.
Following Dean
Sam has followed in Dean's footsteps his entire life, figuratively and literally.
His first steps were following Dean around. His first word was Dad but it was directed at Dean, seeing as he was the only one there to watch him at the time. Sam's first day at school Dean led him to the door, and picked him up at end of day. Whatever Dean did was right; because Dean was a super hero he could never do anything that was bad for Sam. Dean watched out for him, cared for him, protected him. Dean was Sam's: Father, Mother, Brother, Guardian, Best friend, and Idol. Sam wanted to be just like Dean, and he tried his best to be. When Sam realized that he could in fact 'not' be Dean he wanted his brother to be proud of him. Sam tried his best in the category that he excelled at, school. Dean on the other hand wanted to become the best hunter out there, helping people and hunting the thing that had killed Marry; he wanted to make a name for himself. When Dean and his Dad started going on hunts without Sam he felt so alone that he decided that when he grew up he would have lots of people who didn't go on hunts and would be his friends. Eventually, as time passed, this morphed into wanting normal.
When he got acceptance and full ride to Stanford Sam was overjoyed, he was finally going to be just like his big brother. He was going to make a name for himself, even if it wasn't in hunting. He could help people and have a normal life; it was a win-win. The night of the big fight Sam was stunned, he thought that his family would at least try to understand, but he was wrong. "If you walk out that door, don't you ever come back." Speechless and hurt that his family wouldn't want him to have normal and safe, and everything that he wanted in life, he left. Just like Dean had left him so many times before.
At Stanford Sam excelled. He made friends, met new and interesting people, and met the love of his life. Even though he missed Dean like a man in the Sahara desert misses water, or a cool patch of shadow. He had lost his everything that day. When Dean came back joy and elation filled him, after all Dean had come to see him. Dean wanted his help, even though there dad was missing Sam had never felt happier.
After Jess, Dean understood that he needed some time and space, even though Jess died and Cassie only dumped Dean, Dean still understood to some extent. Sam was hell bent on revenging his late love. He wanted nothing more than to kill the Yellow-eyed demon, just like Dean.
When there dad died, selling his soul for Dean, Dean had a very, very hard time. Being the favorite son had to have its downside, and he guessed this was it. They wanted revenge, more now than ever. They kept on hunting. "Helping people hunting things, the family business." The family of two kept fighting even with evil around every corner trying to drag them down.
When Dean decided to sell his soul to 'save' Sam, and in the process go to hell and turn into a demon, Sam followed. When no demon would deal he drank demon blood and used his abilities to send some demons packing.
After all what better way to get to hell to see your brother then killing demons.
