I think Sam is the person that you sort of expect to have feelings for. You meet him and he's just so *nice*. He's careful and smart and so insightful. You would love to talk to him about classic literature, about politics, about the law. You know you'll learn a lot from Sam Winchester and you're so eager to. You fall for him almost immediately and he makes you very, very happy.
Dean is the sort of person that you're surprised to become enamored with. When you meet him, he's that cocky asshole that doesn't give a shit - the person that Dean wants you to see. And for the longest time you just roll your eyes at him and his stupid remarks and his manufactured coolness (though the car is pretty rad). And then you hear him say something. It'll be a comment about Vonnegut or him defending some woman whose boyfriend is getting a little intense. You'll look at him and he'll look at you and you're just left with "Where did that come from". And then you start to notice all the little thing. He's so quick. Sharp. When he feels safe with people, he treats them as if they're made of glass - like he can break them. And he tries so *so* hard. And then one day you look up, and you're in love with Dean Winchester.