That's a good question, I'm curious about it too.
We know he stopped working as a shinigami around 50 years before the current story if I recall ? maybe it was already because of the phantomhives family… or maybe he just got fed up and decided to run away, and then later met the phantomhives…

what if he's in love/fascinated by Vincent ? kind of a parallel to Sebastian and Ciel, where Sebastian is amused/intrigued by Ciel who keeps surprising him ?
At first he thought nothing of it, but Vincent kept making him laugh and he got attached. He couldn't save him, but he was fast enough to at least steal r!Ciel's corpse. The twins look a lot like their father, maybe that's why he got so involved in bringing r!Ciel back, to revive something from the man that brought some fun in his "life"…

But I wonder if that's not the "easy" answer that we're supposed to go for, and then Yana will hit us with another big reveal we didn't see coming ^^'