He only went to the future once, but yeah - then he met the devil wearing Sam's body, while his future self had resisted until the angels decided to fart off. However, they've been given very specific details about when Sam will say yes, both in the future and in the present I believe (iirc, anyway - I think both the devil in the current dude's form and someone in the future mentioned Detroit, "6 months from now" (ofc, around series finale time). Whereas there's been no real indication Dean will ever say yes, besides the angel's insistence that he might as well just give in, because they're gonna keep pestering him until he does.
Also, Sam has a long history of doing things Dean doesn't agree with or otherwise fucking up (leaving his family behind, his "evil destiny, oooo" in season 2, My BFF Ruby, (whcih led to the) demon blood addiction, (and) starting the apocalypse, etc, and it's almost always been presented as something that he could have prevented if he'd just listened to Dean's futile attempts to stop him.
Dean's own fuckups however, have always been presented as noble sacrifices on the show (crossroads deal to go to hell, giving into decades of torture and thus breaking the first seal, etc).