Sam gives up everything to get Dean out of purgatory. His sanity, his life and pretty much everything he worked so hard to get back, to not go down that road. Sam isn't the same Sam and when he breaks open purgatory with his newly honed abilities it completely drains him, he prepared he was going to die but at least he managed to do something right before he did, he just never counted on his big brother still wanting him around and not giving up on him despite what he's become.