I am both an older sibling and a younger one so I have a weird perspective on this.
But first? I want to put on a disclaimer that explains that I've only seen
I'm gonna tackle the "spoiled" issue and start by taking Dean's side looking at Sam. I can easily feel that... resentment. Dean doesn't want to resent him because he knows that Sammy's The Baby and that besides that he really doesn't actually ask for a lot, it's just the fact that all he has to do is ask and he gets it. Dean probably knows that if he asked for anything he would've gotten it too but he probably couldn't bring himself to ask because...
Sam's his little brother and that would feel like taking something away from him. And you know Dean spoiled him rotten too, he totally gave Sammy half of his candy bar even after Sammy ate his. I think that pisses Dean off a bit because he kinda thinks Sam's being ungrateful and feels like Sam maybe even looks down on him for not being "normal" or not wanting to be.
Sam has only known the life he grew up in and therefor he never really liked it because he didn't have a choice, kinda like some kids who are forced to go to church a lot growing up. Dean... it's not that he likes it but it's... One of the first things he remembers is protecting his baby brother. And so his brain cannot begin to comprehend how "Sammy" could know what kind of things are out there and what they do to innocent people and then just... turn his back on protecting them.
Dean is Batman. Something horrible happened to him when he was little and he just does not understand how other people can have things like that happen and not do something about it. Sammy... Is Spider-Man. He doesn't really want people to get hurt but he resents having to be the one to do it. All he wants is to be normal like everyone else in the world and he doesn't think it's fair that he has to do this. He never even got a choice and that pisses him off.
I need Batman!Dean icons now.
This post? Get the SN H/C icon.