Quote toorobbin: Stone didn't leave, he died.
I think Mac's perspective was: The Source of Robin's Abandonment Issues.
Leaving is leaving, in Mac's thesis. He didn't elaborate, notably because Mac knows that Patrick is privy to the facts of how Stone 'left'.
What is the thing that Stone, Jason, and Robin's parents all have in common?
They each left Robin, in some compacity. I think we know what Mac means, if we would just open ourselves up to the possibilities! ;)