Wow with one major exception I seriously disagree.
Let's start there. The tone of the show (minus a few points here and there is fairly bleak. But the line has been for a long time that hunters die young, that their personal lives are fairly crap. And thats average hunters (again all seem to get into the business due to something extremely traumatic occurring to them or their loved ones). Dean and Sam from very much the first episode were shown to be more touched by these PN forces then just a random SPN encounter (say for example what got Bobby's into the life). Their life has always been shown to be harder then the average hunter. So bleak is going to be a very big part of their lives.
On to character of Sam (my favorite character of the show, though Jensen is my favorite of the two actors).
First just because Sam was never the shot first type doesn't mean he cared more. In fact the early season showed on more then one occasion that he didn't care more. He choose to have a safe personal life over helping others. Even when he rejoins Dean is first instinct is revenge or finding Dad (to help lead to the revenge he is seeking) then to help other's.
Sam didn't fully make that jump into needed to save others until season 2's "Hunted". And what was his reason. Because he wanted to selfishly help others just to help others? No. It was because he thought the only way to redeem himself was to save as many people as possible.
Gives you great end results, but the motivations aren't selfless at all.
We see even from the first season (and pre first season ie his reasons to go off and lead a normal life), and second and third season (the seasons you like) that a lot of what motivates him is controlling his own life.
Pre series he hates living the life JOhn forced upon him, he makes his own choice. When he finds out there is a "plam" for him, he seeks what he sees is his own way out of it (ie by saving others to save himself). When Dean's deal threatens to take Dean away we see Sam not take the road Dean says he should take, but one he feels will help save his brother, even at great risk to his own life (seriously willing to turn himself into an immortal organ stealing machine is pretty serious). But all to can some control over his own life. That's really consistent for the character.
THat doesn't mean he doesn't care about random strangers, but it isn't the driving motivation in his life.
And really besides soulless Sam that still has been one of the defining parts of his character.
Dean, who is a womanizer, likes to drink and party has been shown to be the one who actually does care more for the common man. He just doesn't show it on his sleeve. Does he have bad habits, oh god yes. Is his self worth tied down to John's approval and Sam's safety to an extremely codependent level, absolutely.
As for the show saying Dean is good, Sam is bad. With really one exception they have shown both to do bad, and both to do great good.
The one exception is Dean's impact on starting the apocalypse and Sam's role in starting it. Even though the character of Dean does eventually say yes he played his part as well.
But you also have to remember (see it from the character's point of views), Dean's deal and break in hell (which started teh events) were also paid for by a decade of him being a monster to others and him already having to leave with that. And both Sam and Dean (and even Castiel) are all aware of that.
Dean is always going to lash out at Sam for his actions (its in character for him) and he is also going to take part of the blame for them as feelings that he failed his brother. But Dean does not only admit his actions, but we see later that he absolutely forgives Sam as well (as Sam has his had to suffer mightly for his actions).
Next point, ANgels good thus Dean being their vessels means Dean is good. Well cause in point the angels haven't been good (well maybe the gardner), but even Castiel has done many a things (season 4 and 5) that one would classify as being absolutely wrong. And he's the good angel.
The show has made, many, many examples that Angels' aren't good in SPN, and that Dean and Sam's roll were picked long before they were born.
WHy does Dean get rewarded with a family for year, because Sam whats him to have one. Remember Dean is the only character that really doesn't ever have a real normal life (at his age he should have almost no real memories of the events before Azazel seeded Sam). Sam in fact did get his, he had some time to experience a normal life.