[Didn't they seem to have such a healthy relationship tho.]
Perhaps from some objective point of view, I am mistaken to credit this to you. [Truthfully he doesn't care so much about thanking the right person. He has failed as his master's servant for being unable to save Ciel, and for failing to remember his duty while living that "other" life.]
Yet all I know is that you saved my master's life when I was unable to. Were it not for you, my life's purpose would be forfeit. [Healthy levels of devotion and interdependence, yes.]