There is a reason Sam never gives Dean shit about his love for his car. He gets it. Really really gets it. Sam's tried too hard to be normal for too long, he knows what he feels for the car isn't normal. He knows it's there though. He feels it in every part of him when he's away, that deep ache of longing of wanting nothing more than to be back in her, back home.
Dean remembers the place in Lawrence, remembers apple pie for desert and home cooked meals, and Saturday morning cartoons in his pajamas while their mom made breakfast. To Dean that is what home means. The life they had before hunting. It's the main reason Dean didn't want to go back to Lawrence when the poltergeist was haunting the old house. Dean had a home and it was destroyed.
Dean loves his car, she's his baby and he knows her like the back of his hand. He thinks of her both as a beautiful creature, and an extension of himself. He knows that if she were human for even an hour he'd repay her for every wonderful moment he's had with her.
There are things about Baby that Sam doesn't know. Sam doesn't know how to take her apart and put her back together, he doesn't know how to adjust the timing or replace the fan belt. He lets Dean do those things without him because Dean loves it. He knows the certainty that no matter where they are staying she's waiting just outside. He doesn't mind the nights when Dean commanders their hotel room for a tryst. He's fine sleeping in the car. It's been his bedroom for as long as he can remember.
Sam sees the way that Dean looks at her, like she's the only thing in the world that really belongs to him. So he let's him have her all to himself. He doesn't stroke his hands down her sun warmed panels, he doesn't whisper sweet nothings to the dash board. He doesn't even call her Baby anywhere but in his head. He lets Dean keep all those things for himself. But he feels it, maybe more strongly then even Dean himself. The pull of family and a lifetime of bittersweet memories she carries with her.
Sam loves that car, second only to his love for Dean.
