My perception has changed over the years. I remember on my first watch of the early seasons that I (like JBB) thought Dean to be a little too cocky and reckless but with a huge heart and excellent skill to back that up. I found Sam whiny and immature the first time around – especially during the first two seasons.
Since then, every time I watch it again, their characters make more and more sense to me and I love them just the way they are.
I agree with Karen on the assessment of Dean completely.
Sam on the other hand seems just as insecure to me as Dean in many ways. Where Dean wasn’t allowed to be afraid or weak, Sam was so overprotected at the beginning and shielded from the realities of the hunting life that he was very much alone in his struggles to fit anywhere. Dad wanted Sam out of the action for his own protection and Dean wanted Sam out of the hunting life so he would not have to deal and suffer the same way he had to. To me that leaves young Sam very much alone and socially disconnected. It is no surprise to me that Sam in this case became the “researcher”, no one restricted him from reading and learning and he had to spent much time on his own, amusing himself. We know that he always tried to prove his worth to Dean and Dad – like in the flashback in “Just My Imagination” episode – wanting to be part of the family team and being shot down. I can totally understand how the Sam found his value in being the smart one and the educated one, where the other two were leagues ahead in practical skill. Overtime it all evened out a little, of course, but Sam always stayed more of an introvert and attached to learning.
Even if he comes across as slightly cocky or “know-it-all” at times (I honestly think they both do here and there….they ARE two young alpha males after all AND competitive brothers) in the early episodes, I do feel that Sam defers to Dean when it’s important. He still wants to prove himself to Dean.
So, I think, for myself I am gaining more understanding and acceptance for Sam with every rewatch I do, but Dean will always be my first love.