When the dwarves go back to Erebor they fail and become Smaug's slaves. Thorin is his favourite and often claims him, sometimes more brutally than the others. Dwalin is trying to keep an eye on Thorin and can tell his king is starting to crumble and turn into the perfect slave.

http://hobbit-kink.livejournal.com/11476.html?thread=23079636#t23079636