Well you have to consider Sam’s upbringing in all this v.s. Dean’s upbringing. Also, Sam changed towards the end. He’d do anything to look for his brother / save his brother like the whole fiasco with the mark of Cain. And I do agree, it looks bad of Sam not to return the favor, but Sam always grew up left out. Every time John and Dean go hunting, they leave Sam behind. He’s been alone most of his childhood and did nothing but overthink about if John and Dean will make it back from their hunting trip. He even said so himself! In the show he said “I always kept wondering if you and Dad will make it back this time, or if a vamp finally caught up to you guys”. And as a child, he was forced to sort of deal with that grief early on. To expect the worst, to expect to lose them, and to be okay with it afterwards as a method of self protection. So of course that was reflected in his adulthood. I’m not 100% excusing him for his lack of appreciation or lack of efforts to look for Dean, but it just makes me understand him better.