Maybe Sam though opening Purgatory would be putting the earth in danger? So he didn't try to open purgatory because it would release dangerous monsters. Sam was taking safety into consideration and I don't really blame him. Also, I think Sam grieved for Dean for a few months during that year and Amelia was there for him and comforted him during his grief. Maybe he grew and learned to accept that Dean was gone and maybe learned to let him go during that time. I think we are going to see a very independent, grown up Sam who has evolved very much, a Sam who has found his identity without the influence of Dean. And that makes me happy.