Sasuke has forgotten how to cry, somewhere in the eternity between Itachi's betrayal and his own.
He has forgotten how to cry, so he doesn't, even as the pretty dream of a happy life with Naruto and Sakura and Kakashi falls to the ground along with a broken hitei-ate and shatters into nothing but broken could-have-beens. He doesn't cry, even though he would have liked to keep his pipe dream of the person he might have been and the life he might have had, despite only ever having one path to choose from.
Sasuke doesn't cry, because he has forgotten how to. But he wishes he could /remember/, because he wants to mourn the loss of precious things that were never really meant for him to have.
Sasuke can't cry, but the sky still can, and pretend tears will just have to be enough.