The writers really do enjoy giving Dean the shaft when it comes to story arcs don't they lol. Dean's pretty much just become the sidekick of the show, he's just kind of "there" now.
Season 1 had a nice balance of Sam and Dean
Season 2 was mainly about Sam and YEDs plans for him, Dean got to shine at the end by killing YED
Season 3 looked like we'd get a good Dean arc what with his demon deal, but that go over shadowed with the whole Sam and Ruby deal
Season 4 was the same thing, looked like we'd get an awesome Dean arc, but his time in hell got the back burner again to Sam's new powers and Ruby.
Season 5 looked like we'd get an awesome balance of Sam and Dean, and then they gave the whole Michael sword thing to Adam and it was all about Sam and Lucifer
Season 6, well Season 6 was just a complete train wreck so we'll pretend it never happened lol
Season 7 was mostly pretty decent, I actually enjoyed it, but then it turned into the Sam show with his broken head and all. Dean got to shine at the last minute by killing Dick Roman
Season 8 I actually had no complaints with up until now, Dean's time in purgatory could have been fleshed out more, and now yet again we're back to the Sam show with him doing the trials and Dean on sidekick duty.
I don't know, maybe I'm just nitpicking but for me this is a lot of the reason I'm getting sick of Sam, it's just become too much, I think we're way over due for a good Dean story arc.