Cas did suspect that there was more to that story, but assumed Morpheus had a good reason for not wanting to share it. Cas had his own fair share of stories he didn't wish to tell and the last thing he wanted to do was force another to do the same.
"My siblings do not like me," he replied simply. "In fact, they hunt me routinely, but...it's one more thing I've grown accustomed to. I made my choices. You...are not the only disappointment on this ship."
He was tempted to ask Morpheus why that was, out of curiosity, but he refrained from it. Something told him it was not a subject to speak of.
"As for those you miss...I hope you see them here. Or else, I hope you return to them. Whichever you find most preferable."